Fatigue strength of welded structures

Causes of fatigue failure of welded structures

The causes of fatigue failure of welded structures are mainly the following.

  • ① Objectively speaking, the static load bearing capacity of welded joints is generally not lower than that of the parent material; while bearing alternating loads, its bearing capacity is much lower than that of the parent material, and is closely related to the type of welded joint and the form of the welded structure. This is a major factor causing premature failure of some structures due to fatigue of the welded joints.
  • ② Early design of welded structures to static load strength design, no consideration of fatigue resistance design, or fatigue design specifications for welded structures is not perfect, so that there are many welded joints that now seem to be unreasonably designed.
  • ③ Engineering design and technical personnel do not know enough about the characteristics of the fatigue resistance of welded structures, the design of welded structures often copy the fatigue design guidelines and structural forms of other metal structures.
  • ④ Welded structures are becoming increasingly widespread, while the design and manufacturing process artificially blindly pursuing low-cost, lightweight structures, resulting in increasingly large design loads of welded structures.
  • ⑤ Welding structure has a trend toward high-speed heavy-duty direction, the requirements of the welded structure to withstand dynamic load capacity is increasingly high, while the level of scientific research on the fatigue strength of welded structures is relatively lagging behind.

The causes of fatigue failure of welded structures

Static load strength on the fatigue strength of the welded structure

In the study of steel materials, people always hope that the material has a high specific strength, that is, a lighter weight of its own to bear a larger load weight, because the same weight of the structure can have a great load capacity; or the same load capacity can reduce its own weight. So high-strength steel was born, also has a high fatigue strength, the fatigue strength of the base metal always increases with the increase of static load strength.
But for welded structures, the situation is different, because the fatigue strength of welded joints has little to do with the static strength of the base metal, the static strength of the weld metal, the organizational properties of the heat-affected zone and the strength matching of the weld metal, which means that as long as the details of the welded joint are the same, the fatigue strength of high-strength steel and mild steel is the same, with the same S-N curve, and this law is suitable for butt joints, fillet joints Maddox studied the fatigue crack expansion of carbon manganese steel with yield point between 386-636 MPa and the weld metal and heat affected zone welded with six types of electrodes, the results show that: the mechanical properties of the material has a certain effect on the crack expansion rate, but the effect is not significant. In the design of welded structures subjected to alternating loads, trying to meet engineering needs by selecting higher strength steel grades is not meaningful. Only in the case of stress ratios greater than +0.5, static strength conditions play a major role when the welded joint base material should be used high-strength steel.
The reason for the above results is due to the presence of a similar biting edge slag wedge defect along the dissolution line in the joint toe area, its thickness is 0.075mm-0.5mm, the tip radius is less than 0.015mm. the sharp defect is where the fatigue cracking begins, equivalent to the fatigue crack formation stage, and thus the fatigue life of the joint under a certain stress amplitude, mainly determined by the fatigue crack expansion stage . The presence of these defects makes the same type of welded joints of all steels have the same fatigue strength, and the parent material and the static strength of the weld material has little to do with.

The effect of stress concentration on fatigue strength

The effect of the type of joint

Welded joints are mainly in the form of: butt joints, cross joints, T-shaped joints and lap joints, in the joint area due to interference with the transmission line, and thus the phenomenon of stress concentration.
Butt joints have less interference with the force line, so the stress concentration factor is smaller and the fatigue strength will be higher than other joint forms. However, experiments show that the fatigue strength of butt joints varies over a wide range because of a series of factors that affect the fatigue performance of butt joints. Such as the size of the specimen, bevel form, welding method, welding rod type, welding position, weld shape, welding seam processing, post-weld heat treatment, etc. will have an impact on it. Butt joints with a permanent type pad form severe stress concentrations at the pad, reducing the fatigue strength of the joint. Fatigue cracks in such joints are generated from the joint between the weld and the pad, not at the toe, and the fatigue strength is generally equal to that of the most poorly shaped butt joint without a pad.
Cross or T-shaped joints are widely used in welded structures. In such load-bearing joints, the fatigue strength of cross or T-shaped joints is lower than that of butt joints because the stress concentration coefficient is higher than that of butt joints due to the significant cross-sectional changes at the transition from the weld to the base metal. For unbeveled joints connected with fillet welds and open bevel joints with partial penetration welds, fatigue fracture may occur at two weak points when the weld transmits working stress, i.e., at the junction between the base metal and the toe of the weld or on the weld. For cross joints with open bevel weld penetration, fracture generally occurs only at the weld toe, not at the weld seam. The fatigue strength of T- and cross-joints where the weld is not subjected to working stress depends mainly on the stress concentration at the junction of the weld and the main stress plate, with T-joints having higher fatigue strength and cross-joints having lower fatigue strength. The fundamental measure to improve the fatigue strength of T-shaped or cross joints is to open bevel welding, and processing the transition of the weld to make it a smooth transition, through this improvement measure, fatigue strength can be increased substantially.
The fatigue strength of lap joints is very low, which is due to the severe distortion of the force line. The use of the so-called “reinforced” cover of the butt joint is extremely unreasonable, due to increased stress concentration, the use of the cover, the original fatigue strength of the higher butt joint is greatly weakened. In addition, changing the width of the cover or the length of the weld will also change the distribution of stress in the base metal and therefore will affect the fatigue strength of the joint, i.e., as the ratio of the length of the weld to the width of the cover increases, the fatigue strength of the joint increases because the distribution of stress in the base metal tends to be uniform.

Effect of weld shape

Regardless of the joint form, they are connected by two types of welds, butt welds and fillet welds. The weld shape is different and its stress concentration coefficient is not the same, and thus the fatigue strength has a greater dispersion.
The shape of the butt weld has the greatest effect on the fatigue strength of the joint.

  • (1) Effect of transition angle Yamaguchi et al. established the relationship between fatigue strength and the transition angle (outer obtuse angle) between base metal and weld metal. W (weld width) and h (height) were varied in the test, but the h/W ratio remained constant. This means that the pinch angle remains constant, and the test results show that the fatigue strength also remains constant. However, if W is kept constant and the parameter h is varied, it is found that h increases and the fatigue strength of the joint decreases, which is obviously the result of a decrease in the outer nip angle.
  • (2) Effect of weld transition radius The results of Sander et al. showed that the weld transition radius also has an important effect on the fatigue strength of the joint, i.e., the fatigue strength increases as the transition radius increases (the transition angle remains constant).

The shape of the fillet weld also has a large effect on the fatigue strength of the joint.
When the calculated thickness of a single weld a and the ratio of plate thickness B a/B < 0.6 to 0.7, generally broken in the weld; when a/B > 0.7, generally broken in the base metal. But increasing the size of the weld to improve fatigue strength is only effective within a certain range. Soete, VanCrombrugge used a 15mm thick plate with different fillet welds and found that the fracture occurred in the base metal at the toe or in the weld when the foot of the weld was 13mm. When the toe of the weld was less than this value, fatigue fracture occurred in the weld; when the toe size was 18 mm fracture occurred in the base metal. Accordingly, they propose the limit foot size: S = 0.85B. S is the foot size and B is the plate thickness. It can be seen that even if the weld foot size up to the plate thickness (15mm), the fracture results can still be obtained at the weld, and this result is in good agreement with the theoretical results.

Effect of welding defects

There are a large number of different types of defects in the weld toe area, and these different types of defects lead to early cracking of fatigue cracks and a sharp drop in the fatigue strength of the base material (down to 80%). Welding defects can be broadly divided into two categories: faceted defects (such as cracks, unfused, etc.) and volumetric defects (porosity, slag, etc.), their degree of impact is different, while the impact of welding defects on the fatigue strength of the joint with the type, direction and location of defects.

  • 1) Cracks in welding, such as cold and hot cracks, in addition to being accompanied by a brittle tissue structure, is a serious source of stress concentration, which can significantly reduce the fatigue strength of the structure or joint. Earlier studies have shown that in a mild steel butt joint specimen 60 mm wide and 12.7 mm thick, with cracks 25 mm long and 5.2 mm deep in the weld (they account for about 10% of the cross-sectional area of the specimen), under alternating load conditions, the fatigue strength of its 2 × 106 cycle life is reduced by about 55% to 65%.
  • 2) Unwelded penetration It should be noted that unwelded penetration is not necessarily considered a defect because sometimes some joints are artificially required to be perimeter welded, typically in the design of certain pressure vessel receivers. Not weld through defects are sometimes surface defects (single-sided weld), sometimes internal defects (double-sided weld), it can be local in nature, can also be the overall nature of the. Its main effect is to weaken the cross-sectional area and cause stress concentration. Fatigue life at 10% weakened area compared to the fatigue strength of the test results without this type of defect is reduced by 25%, which means that its effect is not as serious as cracking.
  • 3) Unfused is extremely rare to date because of the difficulty of preparing specimens. However, there is no doubt that unfused is a flat defect and therefore should not be ignored and is generally treated as the same as unbroken weld.
  • 4) Bite edge characterization of the main parameters of the bite edge length L, bite edge depth h, bite edge width W. The main parameter affecting the fatigue strength is the bite edge depth h, currently available depth h or depth to plate thickness ratio (h/B) as a parameter to assess the fatigue strength of the joint.
  • 5) Pores are volumetric defects, and Harrison’s analysis of the previous test results concluded that the fatigue strength decrease is mainly due to the reduction of the cross-sectional area size by pores, and there is a linear relationship between them. However, some studies have shown that when the surface of the specimen is machined so that the pores are on the surface or just below the surface, the adverse effect of the pores increases, and it will act as a source of stress concentration and become the cracking point of fatigue cracks. This indicates that the location of the pore has a greater effect on the fatigue strength of the joint than its size, and the surface or subsurface pore has the most adverse effect.
  • 6) Slag IIW research reports indicate that, as a volumetric defect, slag has a greater impact on the fatigue strength of the joint than porosity.

Through the above introduction can be seen welding defects on the fatigue strength of the joint, not only with the size of the defect, but also determined by many other factors, such as surface defects than internal defects, perpendicular to the direction of the force of the impact of facial defects than other directions; located in the residual tensile stress area of the impact of defects than in the residual compressive stress area; located in the stress concentration zone defects (such as weld toe cracks) than In the uniform stress field in the same defect impact is large.

The effect of welding residual stress on fatigue strength

Welding residual stress is a characteristic of the welded structure, therefore, it is the impact of the fatigue strength of the welded structure is a widespread concern, for which a large number of experimental studies have been carried out. Tests are often used with welding residual stress specimens and after heat treatment to eliminate the residual stress specimens, fatigue tests for comparison. As the generation of welding residual stress is often accompanied by welding thermal cycle caused by changes in material properties, and heat treatment in the elimination of residual stress while also restoring or partially restoring the performance of the material, but also due to the dispersion of the test results, so the test results have produced different interpretations, the impact of welding residual stress also has a different evaluation.
Try to cite the early and recent research work carried out by a number of people as an example, can clearly illustrate the problem, with residual height of the butt joint for 2 × 106 cycle test results, different researchers have come to different conclusions. Some people found that the fatigue strength of the heat treatment stress relief specimen increased by 12.5% compared to the fatigue strength of the same specimen in the welded state; others found that the fatigue strength of the welded and heat treated specimens was the same, i.e., the difference was not significant; however, others found that the fatigue strength increased after the use of heat treatment to eliminate residual stress, but the increase was much less than 12.5%, etc. The same is true for the test results of surface polished butt joint specimens, i.e., some tests suggest that the fatigue strength can be increased by 17% after heat treatment, but some test results indicate that the fatigue strength is not increased after heat treatment, etc. This problem has long been confusing, until some scholars in the former Soviet Union conducted a series of tests under alternating load, only gradually clarified the issue.
One of the most worthy of presentation is Trufyakov in different stress cycle characteristics of the welding residual stress on the fatigue strength of the joint. The tests were conducted using 14Mn2 ordinary low-alloy structural steel with a transverse butt weld on the specimen and one longitudinal weld channel each on the front and back side. One group of specimens was heat treated for residual stress relief after welding, and the other group was not heat treated. Fatigue strength comparison test using three stress cycle characteristic coefficient r = -1, 0, +0.3. under alternating load (r = -1), the fatigue strength of the elimination of residual stress specimens close to 130MPa, while only 75MPa without elimination of residual stress, under pulsating load (r = 0), the fatigue strength of the two groups of specimens are the same, 185MPa. and when r = 0.3, by The fatigue strength of the heat-treated specimens with residual stresses removed was 260 MPa, which was instead slightly lower than that of the unheat-treated specimens (270 MPa). The main reason for this phenomenon is that when the r value is high, for example, under pulsating load (r=0), the fatigue strength is higher, and the residual stress is released faster under the higher tensile stress, so the effect of residual stress on fatigue strength is weakened; when r increases to 0.3, the residual stress is further reduced under the load, which actually has no effect on fatigue strength. The heat treatment eliminates the residual stress and softens the material at the same time, thus making the fatigue strength decrease after heat treatment. This test is a good illustration of the residual stress and welding thermal cycling caused by material changes on the fatigue strength. From here we can also see the impact of welding residual stress on the fatigue strength of the joint with the fatigue load of the stress cycle characteristics. That is, the effect is greater when the cyclic characteristic value is lower.
As pointed out earlier, because of the residual stresses in the structural weld that reach the yield point of the material, the actual stress cycle applied near the weld in a joint with a constant amplitude applied stress cycle will oscillate downward from the yield point of the material, regardless of the cyclic characteristics of its original action. For example, if the nominal stress cycle is +S1 to -S2, the stress range would be S1+S2. However, the actual stress cycle range in the joint would be from Sy (stress amplitude at the yield point) to Sy-(S1+S2). This is very important when studying the fatigue strength of welded joints, and it has led some design codes to replace the cyclic characteristic r with the stress range.
In addition, the size of the specimen, the loading method, the stress cycle ratio, and the load spectrum also have a significant effect on the fatigue strength during the test

Process methods to improve the fatigue strength of welded structures

Fatigue cracks in welded joints are generally found in the root and toe of the two parts, if the risk of fatigue cracks in the root area is suppressed, the danger of welded joints are concentrated in the toe area. Many methods can be used to improve the fatigue strength of welded joints.

  • ① Reducing or eliminating weld defects, especially open defects.
  • ② Improving the geometry of the weld toe area to reduce the stress concentration factor.
  • ③ Regulating the weld residual stress field to produce a residual compressive stress field. These improvement methods can be divided into two main categories, as shown in Table 1.

Welding process optimization methods are considered not only for improving the fatigue strength of the welded structure, but also for the static load strength of the welded structure, the metallurgical properties of the welded joint and other aspects are of great benefit, and there is a lot of information in this regard is not repeated here.

Table.1 improvement methods of fatigue strength of welded structure

Improvement method of fatigue strength of welded structure Welding process optimization Local geometry Quality Control Control of welding defects 1
Improvement of geometry 2
Technological process Welding sequence 3
Residual stress (< 0) Metallurgical treatment of weld toe 4
Weld bead modeling Weld toe geometry 5
Metallurgical and metal state 6
Weld improvement Local geometry Machining Weld toe grinding 7
Water impact 8
Local remelting TIG Dressing 9
Plasma repair 10
Residual stress Stress release method Heat treatment 11
Mechanical treatment 12
Local heating 13
Mechanical method Mechanical contact Shot peening 14
Hammering 15
Ultrasonic impact 16
Welding Stamping 17
Local compression 18

The following is a detailed discussion of the main methods for improving the fatigue strength of welded joints in three parts, considered from the perspective of process methods.

Improvement of weld toe geometry to reduce stress concentration methods

1) TIG repair
Domestic and foreign research has shown that TIG melt repair can significantly improve the fatigue strength of welded joints, this method is to use tungsten argon arc welding method in the transition part of the welded joint remelted once, so that the weld and the base metal between the formation of a smooth transition. The stress concentration is reduced, and the tiny non-metallic slag in the area is also reduced, thus increasing the fatigue strength of the joint.
The melt repair process requires that the torch is generally located 0.5 to 1.5 mm from the toe area and that the remelting area is kept clean, better if it is lightly polished beforehand. The important thing is to remelt in the arc, how to deal with the re-arc method, because this will inevitably affect the quality of the remelted weld channel, generally recommended re-arc is the best position in front of the arc pit 6mm, recently the International Institute of Welding some European countries and Japan’s Institute of Welding, the use of unified by the British Institute of Welding prepared specimens for – some It has been higher than the highest FAT value of the joint details fatigue strength of the International Welding Institute.
2) Mechanical processing
If the surface of the weld is mechanically processed, the degree of stress concentration will be greatly reduced and the fatigue strength of the butt joint will be increased accordingly. When the weld is free of defects, the fatigue strength of the joint can be higher than the fatigue strength of the base metal. However, the cost of this surface mechanical processing is very high, so only really beneficial and can indeed be processed to the place, it is suitable for the use of such processing. The weld with serious defects and without bottom welding, its defects or weld root stress concentration is much more serious than the stress concentration on the surface of the weld, so in this case the mechanical processing of the weld surface is meaningless. If there is an unperforated defect, because the fatigue crack will not start cracking at the residual height and toe, but move to the root of the weld unperforated. In the presence of an open defect, machining tends to reduce the fatigue strength of the joint.
Sometimes the overall weld metal is not machined, but only the toe is machined and ground, which can also significantly improve the fatigue strength of the joint. Research shows that in this case, the cracking point is not at the toe, but transferred to the weld defect site.
Makorov, the Soviet Union on the fatigue strength of high-strength steel (tensile strength σb = 1080MPa) transverse butt weld of alternating load test shows that the fatigue strength of ± 150MPa at 2 × 106 cycles under weld conditions, if the weld is mechanically processed to remove the residual height, the fatigue strength increased to ± 275MPa, which has been comparable to the fatigue strength of the base metal. However, if the local grinding process is carried out at the weld toe, the fatigue strength is ±245MPa, which is 83% of the effect of machining, and the fatigue strength is increased by 65% compared with the weld state. Of course, regardless of whether the machining method or the grinding method is used, if the process cannot be carried out carefully as required in order to ensure the processing effect, the increase in fatigue strength is limited.
3) Grinding with grinding wheels
The use of grinding wheel grinding, although its effect is not as good as mechanical processing, but also an effective way to improve the fatigue strength of welded joints. The International Institute of Welding recommends the use of high-speed electric or hydraulic driven grinding wheel, the speed of (15000 ~ 40000) / min, grinding wheel made of carbon-tungsten material, its diameter should ensure that the grinding depth radius should be equal to or greater than 1/4 of the plate thickness. A recent study by the International Welding Institute shows that the nominal fatigue strength of the specimen under 2×106 cycles is increased by 45% after grinding, and if the obtained nominal fatigue strength of 199 MPa is converted to the corresponding characteristic value (135 MPa) it is also higher than the highest FAT value in the joint detail fatigue strength of the International Welding Institute. It is important to note that the grinding direction should be the same as the direction of the force line, otherwise an indentation perpendicular to the force line will be left in the weld, which is equivalent to a source of stress concentration and serves to reduce the fatigue strength of the joint.
4) Special welding rod method
This method is the development of a new type of welding rod, its liquid metal and liquid slag has a high soluble wetting capacity, which can improve the transition radius of the weld, reduce the angle of the weld toe, reduce the degree of stress concentration at the toe, thereby improving the fatigue strength of the welded joint. Similar to the disadvantages of TIG fusion repair, it has a strong selectivity for the welding position, especially suitable for flat welding position and flat fillet welding, while for vertical, horizontal and supine welding, its superiority is significantly reduced.

Methods for adjusting the residual stress field to produce compressive stress

1) Pre-overload method
If a tensile load is applied to the specimen containing the stress concentration until yielding occurs at the notch, accompanied by a certain amount of tensile plastic deformation, after unloading, the notch and the nearby tensile plastic deformation will produce compressive stresses, while in other parts of the cross-section of the specimen will be balanced with the tensile stress below the yield point. The specimen subjected to this treatment will have a significantly smaller stress range in its subsequent fatigue test than the original specimen without pre-overload, so it can improve the fatigue strength of welded joints. The results of the study show that large welded structures (such as bridges, pressure vessels, etc.) need to be subjected to certain pre-overload tests before they are put into operation, which is beneficial for improving fatigue performance.
2) Local heating
The use of local heating can regulate the welding residual stress field, that is, the stress concentration in the compressive residual stress, and thus is beneficial to improve the fatigue strength of the joint. This method is currently limited to longitudinal discontinuous welds, or joints with longitudinal reinforcement plates.
For single-sided angle joint plate, the heating position is generally about 1/3 of the plate width from the weld, for double-sided angle joint plate situation heating position for the center of the plate. This ensures that compressive stresses are generated in the weld, which can improve the fatigue strength of the joint. The results obtained from the application of this method vary from 145-150% improvement in fatigue strength for single-sided corner jointed plates to 70-187% improvement for double-sided corner jointed plates.
The local heating position has an important effect on the fatigue strength of the joint, when spot heating is carried out at both ends of the weld, then the compressive residual stress is induced at the notch of the weld end, resulting in a 53% increase in fatigue strength; however, when spot heating is carried out at the center of the specimen at the weld end, the distance from the weld end is the same, which produces the same effect on the metallographic organization, but since the residual stress is tensile residual stresses, the measured fatigue strength of the joint is the same as that of the non-treated specimen.
3) Extrusion method
The mechanism of local extrusion is the same as that of spot heating, i.e., both rely on compressive residual stresses to improve the fatigue strength of the joint. However, the point of action is different, and the extrusion position should be located at the position where the residual compressive stress is required. The effect of the extrusion method on high-strength steel specimens is more significant than that on mild steel.
4) Gurnnert’s method
As it is sometimes difficult to determine the exact heating location and heating temperature of the local heating method, in order to obtain satisfactory results, Gunnert proposed a method, the main point of the method is to directly to the notched part rather than the nearby part of the heating to produce plastic deformation but below the phase change temperature of 55 ℃ or 550 ℃, and then sharply sprayed cooling. Since the metal under the surface and its surrounding metal not subject to spraying cools later, when it cools, the shrinkage will produce compressive stress on the cooled surface. By this compressive stress, the fatigue strength of the component can be improved. It is important to note that the heating process should be slow in order for the bottom layer to also be heated, with Gunnert recommending a heating time of 3 min and Harrison recommending a heating time of 5 min.
Ohta used this method to successfully prevent fatigue cracks inside the buttress piping. This was done by heating the outside of the pipe by induction and cooling the inside with circulating water. As a result, compressive stresses were generated inside the pipe, thus effectively preventing fatigue cracks from forming inside the pipe. After the treatment, the fatigue crack expansion rate of the butt weld pipe is greatly reduced, reaching the same crack expansion rate as that of the base material.

Methods to reduce stress concentration and generate compressive stresses at the same time

1) Hammering method
The hammering method is a cold working method that works by causing compressive stresses on the surface of the weld toe of the joint. Therefore, the effectiveness of this method is related to the plastic deformation produced on the surface of the weld toe; hammering also reduces the sharpness of the notch present, thus reducing the stress concentration, which is the reason for the significant increase in the fatigue strength of the joint. Recent work by the International Welding Institute shows that for non-load bearing T-joints, the fatigue strength of the joint is increased by 54% after 2×106 cycles of hammering.
2) Shot peening
Shot peening is another form of hammering, and is also a method of impact machining. The effect of shot blasting depends on the diameter of the shot size, which should not be too large to enable it to deal with minor defects. At the same time, the size of the shot should not be too small to ensure a certain cold hardening properties, the shot can generally act on the surface at a depth of a few thousandths of a millimeter. Research results show that shot peening can significantly improve the fatigue strength of high-strength steel joints, shot peening has outstanding effects on argon arc welding high-strength steel materials, the extent of which is even higher than TIG fusion repair. At the same time, TIG fusion repair with shot peening hammering, the effect is more significant.

The latest technology to improve the fatigue strength of welded joints

Ultrasonic impact treatment method

Ultrasonic impact developed in recent years to improve the fatigue strength of welded joints and structures, the mechanism and hammering and shot blasting is basically the same. But this method actuator lightweight, controllable, flexible and easy to use, very low noise, high efficiency, less restricted application, low cost and energy saving, applicable to a variety of joints, is an ideal method to improve the fatigue properties of welded joints after welding. Several typical welded structural steel butt and non-load longitudinal angle joint implementation of ultrasonic impact treatment, and then the welding state and impact treatment of the comparative fatigue test, the study of the actual effect of ultrasonic impact method to improve the fatigue strength of the welded head, the comparative results are shown in Table 2. can be seen, the fatigue strength of the welded joint by ultrasonic impact treatment, fatigue strength increased by 50 ~ 170%, the effect is very significant.
Table.2 Comparison of fatigue strength before and after ultrasonic impact treatment

Material and joint form Fatigue strength Ds / MPa Degree of improvement %
Weld state Impact processing state
Q235B (R=0.1)-butt joint 152 230 51
SS800 (R=0.05) – Docking 306 101
16Mn (R=0.1) – Butt 285 88
Q235B (R=0.1)-longitudinal angle joint 104 200 92
SS800 (R=0.05) – Longitudinal corner connection 279 168
16Mn (R=0.1) – Longitudinal angle joint 212 104

Low phase change spot welding rod method

Improve the fatigue strength of welded joints principle and development

Compressive stress can improve the fatigue strength of welded joints and has been discussed in a large amount of literature, however, the question is how to introduce compressive stress in welded joints more easily.
It is known that due to the different chemical composition, alloy content and cooling rate, the steel material will undergo different tissue transformation or multiple tissue transformation during cooling, this tissue transformation is accompanied by volume expansion, which will produce phase change stresses under constrained conditions, which are compressive stresses. For the weld metal, this will facilitate the reduction of the residual tensile stress or even the emergence of residual compressive stress, thus improving the mechanical properties of the welded joint. Low Transformation Temperature Welding Electrode (LTTE) is a new type of welding material that uses phase transformation stresses to produce compressive stresses in the welded joint to improve the fatigue strength of the welded joint.
As early as the 1960s, the former Soviet Union welding experts proposed the low phase transformation point welding electrode method can improve the fatigue strength of welded structures, but at that time did not propose the concept of “low phase transformation point welding electrode”, only called it a special welding electrode whose overlay metal composition relies on 3-4% Mn content to reduce the phase transformation point, to achieve metallurgical phase transformation. The literature points out that the fatigue strength of these special electrodes is 75% higher than the fatigue strength of the unexfoliated test when fatigue tests are performed on small specimens with these electrodes.
In recent years, relying on Cr and Ni to reduce the martensitic phase change point of the weld material deposited, and due to the development of ultra-low carbon steel materials, low phase change point welding rod has been developed rapidly, Japan and China in this regard a lot of research, but is still in the laboratory stage.

Effectiveness of LTTE welding rod to improve fatigue strength

Tianjin University School of Materials designed and optimized the development of low phase change spot welding rod, and a large number of fatigue tests and process performance tests on a variety of welded joints.
(1) LTTE method
Low phase change spot welding rod LTTE and ordinary welding rod E5015 were used to weld transverse butt joints, non-load cross joints, longitudinal wrap around fillet joints, longitudinal parallel fillet joints and longitudinal butt joints, and fatigue comparison tests. The results show that the fatigue strength of LTTE joints with phase change spot welds increased by 11%, 23%, 42%, 46% and 59% respectively compared to the fatigue strength of E5015 joints with ordinary welds, and the fatigue life increased from several times to hundreds of times.
Table.3 Improvement in fatigue strength of different types of welded joints

Electrode type Transverse butt joint Non load bearing cross joint Longitudinal circumferential fillet weld joint Longitudinal parallel fillet weld joint Longitudinal butt joint
E5015 welding rod 176.9 202.1 167 182.7 179.4
LTTE welding rod 157.8 164.8 118.3 124.9 113
Degree of improvement 11% 23% 41% 47% 58%
Degree of stress concentration Easy K1 Medium K2 Strong K3 Special strong K4 Special strong K4
Degree of constraint Small Big

As the low phase change spot welding rod is the residual compressive stress obtained by the volume expansion of the martensite phase change at a lower temperature, the size of the residual compressive stress has a greater relationship with the degree of constraint of the welded joint, the greater the degree of constraint, the greater the residual compressive stress, the greater the effect of fatigue strength improvement.
(2) low phase change point welding rod welding toe fusion repair (LTTE-dressing) method
However, in order for the weld metal to undergo martensitic phase transformation at normal cooling rates and at lower temperatures, more alloying elements are added to the weld material, which makes the cost of low phase transformation spot welding materials much higher. If all the welds of a welded structure are applied using low phase change welding materials, the cost of the welded structure will also increase significantly, which is very uneconomical.
It is known that the fatigue fracture of the welded joint mainly from the toe area cracking, if the welded joint toe area residual compressive stress, the fatigue strength of the welded joint can be improved, and do not need to use all low-phase change spot welding rod, which can reduce the cost of use. Considering this idea, Tianjin University proposed a low phase change spot electrode welding toe fusion repair (LTTE-dressing) based on experiments to improve the fatigue strength of welded joints. The fatigue strengths of LTTE-dressing and ordinary welded joints were compared using two types of joints: non-load bearing cross joints and longitudinal wrap-around fillet joints, and the fatigue strengths of the former were increased by 19.9% and 41.7%, respectively, which proved the feasibility and practicality of this idea, and provided a more reasonable application of LTTE for low phase change spot welders in The practical application of low-phase change spot welding rod LTTE in engineering has been studied in preliminary experiments, while low-phase change spot welding rod weld toe fusion repair (LTTE-dressing) joints can also reflect the application of low-phase change spot welding rod in cover welds and near toe cover welds.

Advantages and disadvantages of low phase change spot welding electrodes

Advantages

  • (1) The low phase change spot welding method is carried out simultaneously with the welding process, avoiding the inconvenience of post-weld processing.
  • (2) The low phase change spot welding method requires no special operational requirements and is therefore simple and easy to operate.
  • (3) The low phase change spot welding material is used to improve the fatigue strength of welded joints, and since it is not affected by the thermal action of the subsequent weld channel, it is more suitable for fatigue strength improvement of concealed welds, covered welds, back welds of single-sided welds, and other welds that cannot be processed after welding.
  • (4) LTTE welding rod can also be used for the repair of fatigue cracks in welded structures.

Disadvantages
More alloying elements are added to the weld material, thus making the cost of low phase change point welding material increases, but can be compensated by methods such as LTTE-dressing.

Conclusion

In summary, it can be seen that in recent years, due to the development of welded structures in the direction of high-speed heavy load, the requirements for its ability to withstand dynamic loads are increasingly high, so the development and promotion of the application of new technologies to improve the fatigue properties of welded joints is of great significance in promoting the application of welded structures. Relatively speaking, the latest development of ultrasonic impact technology at home and abroad, as well as the use of low phase change point welding materials to improve the fatigue strength of welded joints is an important research direction to improve the fatigue performance of welded structures technology and process.

Source: China Welded Pipe Manufacturer – Yaang Pipe Industry Co., Limited (www.steeljrv.com)

(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)

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