What is pearlitic steel
What is pearlitic steel?
Pearlitic steel is also called pearlitic heat-resistant steel. This kind of steel has pearlite and ferrite microstructure in normalized state. The steel has low alloy element content, good process performance, and the maximum working temperature can reach 600 ℃. According to the purpose, this kind of steel can be divided into boiler tube steel, gas ladle steel, fastener steel and rotor steel.
In the welding process of pearlite steel joints, especially in the process of heat treatment and high-temperature operation, there is diffusion and migration of carbon, resulting in decarburization layer in low chromium steel and carburization layer on one side of adjacent high chromium steel. When heated for a long time at high temperature, the base metal of decarburization layer will be softened due to the reduction of carbon element, and the pearlite structure will become ferrite structure, At the same time, it promotes the grain growth at the decarburization layer and forms a coarse grain crystalline layer along the fusion zone.
In addition to melting the carbon into the parent material, the remaining carbon element precipitates in the form of chromium carbide to soften the structure.
When the chromium content in the weld metal increases from 0.6% to 5%, the influence on the width of decarburization layer of low carbon steel base metal is the most significant, while further increasing the chromium content will reduce the influence. When the chromium content in the weld metal increases to 25%, the width of decarburization layer decreases significantly, and the width of carburization layer in the weld metal is also reduced. A certain amount of carbide forming elements (such as Cr, Ti, W, V, Nb, etc.) in pearlite base metal can significantly reduce the diffusion and migration of carbon. If the migration of carbon is too large, it can be shown by slight corrosion. Under the microscope, there is a white bright low carbon band in the heat affected zone of carbon, while there is a dark high carbon zone in the weld metal of stainless steel.
Thermal stress is an important reason affecting joint strength and thermal fatigue. The linear expansion coefficient of austenitic steel is 30% ~ 50% larger than that of pearlitic steel, and the thermal conductivity is only 1 / 3 of that of pearlitic steel. The joints of two materials will produce thermal stress in the fusion zone during post weld cooling, heat treatment and use. Thermal stress is an important reason affecting joint strength and thermal fatigue.
Pearlitic steel dissimilar joints bear serious thermal alternating stress when working under periodic heating and cooling conditions. As a result, thermal fatigue cracks occur along the fusion zone on one side of pearlitic steel and expand along the weakened decarburization layer, resulting in poor strength and toughness of the joint.
Selection of welding materials for pearlitic steel
Pearlitic steel welding generally follows the following principles to select welding materials:
- (1) It can overcome the adverse effect of pearlite steel on the dilution of weld metal.
- (2) Restrain the adverse effects of carbide forming elements and ensure the service performance of the joint, including mechanical properties and comprehensive properties.
- (3) The welded joint shall not produce cold and hot cracks.
- (4) Good process performance and high production efficiency to reduce the cost as much as possible. According to the expected service conditions of welded joints, appropriate filler metal shall be selected after considering the influence of dilution on weld metal composition.
For medium temperature operation, even when the service temperature is lower than 427 ℃, austenitic stainless steel filler metal is generally not used, but Ni Cr Fe filler metal is used. During multi pass welding, a variety of filler metals can be used according to the changes of each weld pass. Q235 steel is used as welding material for austenitic stainless steel welding.
When stainless steel filler metal is used, in order to produce austenite and ferrite structure in the weld metal on the pearlite side, reduce the fusion zone and reduce the weld plasticity, the austenitic electrode with high nickel content can be used for surfacing welding on the groove surface of pearlite steel, and then processed, and then the austenitic electrode with low nickel content can be used for welding the joint, which is used for the dissimilar joint between pearlite steel and austenitic steel above 371 ℃, Nickel alloy is usually used as filler metal. This kind of filler metal is used for transition joints working in periodic temperature changes. It has the following advantages: it can allow a variety of base metals to be diluted without producing crack sensitive structures; The low solubility of carbon can reduce the migration of carbon from low alloy steel to the crack. When the selected nickel alloy filler metal (such as ERNiCr-3) has a linear expansion coefficient close to that of low alloy steel (such as 2). 25cr-1mo) base metal, the stress generated at the weld interface is much smaller than that when austenitic stainless steel is used as filler metal. At the same time, the metal at the interface has sufficient oxidation resistance and high creep fracture strength, which can ensure the strength of appropriate joints in use.
Welding process and post weld heat treatment of pearlitic steel
When welding pearlitic steel in dissimilar joints, in order to reduce the fusion ratio and reduce the dilution of weld metal, processes such as large groove, low current, fast and multi-layer welding should be adopted. Due to the different expansion coefficients, the stress distribution can be changed with the help of appropriate system design and joint arrangement, and the long weld should be welded in sections.
The selection of heat treatment process for dissimilar austenitic steel welded joints depends on the grade of steel, the shape of components and working conditions. When welding general heat-resistant steel that does not need to eliminate welding stress and works under medium temperature conditions, post weld heat treatment may not be carried out. For components whose manufacturing requirements and service conditions specify that welding stress should be eliminated, stabilization treatment at 800 ~ 850 ℃ is sufficient. For components working at high temperature, it is better to austenitize welded components at 1100 ~ 1150 ℃. However, if intergranular corrosion resistance is required, the austenitizing temperature shall not exceed 1050 ℃. If dispersion strengthened austenitic steel is used in the welded joint, heat treatment must be carried out after welding to restore the properties of the near seam area. At this time, the selection of heat treatment process is usually carried out according to the requirements of dispersion strengthened steel.
For the welding rod, preheating and post weld heat treatment of pearlitic steel, if there is a steel with high hardenability tendency in the welded dissimilar steel, appropriate preheating must be carried out. The dissimilar joints of pearlitic steel and austenitic steel with high hardenability tendency also need post weld heat treatment to prevent hardening structure, reduce welding residual stress and prevent cold cracks. Due to the embrittlement and diffusion layer of the above welded joints in the fusion zone, especially when welding large thickness rigid components, the weldment may have brittle fracture in the fusion zone during tempering treatment or later use. Therefore, high nickel electrode can be used for surfacing welding on the groove surface of pearlite steel first, and then welding. Due to the different expansion coefficients between pearlitic steel and austenitic steel, great residual stress will be generated at the joint after welding. The external load acting on the joint can be reduced with the help of appropriate system design and joint arrangement, and the transition layer can be added when necessary.
Microstructure and properties of pearlitic steel welded joints
Pearlitic steel welded joints are divided into three main characteristic zones: weld zone, fusion zone and heat affected zone. When austenitic steel electrode is used, the weld structure is austenite and a small amount of skeleton ferrite. The fusion zone is acicular structure and “white bright” zone which is not easy to be corroded; Near the fusion zone is the heat affected zone with coarse structure. The microhardness test shows that the fusion zone is a high hardness zone.
The dilution degree of weld metal of pearlitic steel is affected by welding method, joint form, welding process parameters (welding current, welding speed), preheating temperature, welder operation technology and other factors. Due to dilution, arc convection and mechanical stirring, the weld metal is a uniform mixing zone between austenitic steel electrode and pearlite base metal. The dilution degree of base metal to filler metal is also different with different groove forms and welding processes.
The chemical composition of the welding metal can be calculated according to the filler metal, base metal composition and fusion ratio. The weld microstructure can be predicted according to the Schaeffler weld microstructure diagram. In fact, the chemical composition of the middle part of the weld is very different from that of the weld edge. The edge of the molten pool is close to the solid base metal. The liquid metal has low temperature, poor fluidity, short liquid residence time and weak mechanical stirring. It is a retention layer. The molten base metal and filler metal can not be fully mixed, and the closer to the fusion zone, the greater the proportion of base metal composition.
The Cr and Ni elements in pearlitic steel weld diffuse to the molten base metal, and the carbon elements in the base metal diffuse to the weld due to the affinity of Cr, and finally form an alloy element concentration gradient. Composition distribution of alloying elements near the fusion zone between No. 20 steel and cr25ni20 (A402). Due to the high content of Cr and Ni in the weld, it meets the content required for single-phase austenite in the schaffler weld structure diagram, so the Cr and Ni in the austenite structure fusion transition zone are not enough to form single-phase austenite, and brittle martensite may be formed during rapid cooling.
Composition distribution of alloying elements near the fusion zone between Cr5Mo steel and cr25-ni13 (A302). The change of alloy element concentration will inevitably lead to the change of microstructure and form a transition zone called fusion zone. Although the transition zone is very narrow, it has an important impact on the mechanical properties of welded joints.
Electron probe microanalysis results of C and Cr elements on both sides of the fusion zone between austenitic weld and low carbon steel in the as welded state and after high temperature heating treatment. Obviously, after 6000 ℃ × After 100 h high temperature heating treatment, the carbon content near the weld metal side of the welding fusion zone increases significantly, resulting in significant changes in the microstructure and properties near the fusion zone, especially the reduction of impact.
Cr is a strong carbide forming element. After carbon atoms diffuse and migrate from the weld to the fusion zone along the crystal edge with low activation energy, C element forms a stable carbon compound Cr23C6. Due to the slow dissolution of carbides in the fusion zone and subsequent diffusion to the weld gap, an obvious decarburization layer is formed. Increasing the content of chromium or ferritic elements in the weld will increase the width of decarburization layer.
Ni is an austenitizing element, which will increase the activity coefficient of carbon, reduce the chemical stability of carbide, and weaken the binding ability of carbide forming elements to carbon. The width of the fusion transition zone is mainly affected by the welding process and the chemical composition of the filler metal. For example, the width of the fusion zone, especially the width of martensite layer, can be reduced by using high current and high Ni content electrodes.
When the heterogeneous joint of pearlite steel works below 425 ℃, the joint welded with 25-13 filler metal has good performance; When the temperature is above 425 ℃, brittle zone will be generated near the pearlite easy side of the fusion zone, resulting in the fracture of the joint along the fusion line. Therefore, when the heterogeneous joint between pearlite steel and austenitic steel is above 425 ℃ or under the environment with large changes in temperature and pressure, filler metal with nickel content greater than 25% (such as A507) or even pure Ni based filler metal shall be used, The width of the low plastic band in the fusion zone is minimized to ensure the strength and corrosion resistance of the joint.
Source: China Pipe Fittings 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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