Classification and heat treatment of stainless steel
There are many kinds of classification methods for stainless steel, such as chemical composition, functional characteristics, metallographic structure and heat treatment characteristics. Considering from the aspect of heat treatment, it is more practical to divide according to the metallographic structure and heat treatment characteristics.
Types of stainless steel
The main alloy element is Cr, or a small amount of stable ferrite elements, such as Al, Mo, etc., are added, and the structure is ferrite. The strength is not high, the property can not be adjusted by heat treatment, it has certain plasticity and brittleness. It has good corrosion resistance in oxidizing medium (such as nitric acid) and poor corrosion resistance in reducing medium.
In order to further improve the corrosion resistance, it is necessary to add Mo, Cu, Si, Ti, Nb and other elements. It has low strength, high plasticity and high toughness. It has strong anti-corrosion ability to oxidizing medium, and has better anti intergranular corrosion ability after adding Ti and Nb.
Martensitic stainless steel mainly contains 12-18% Cr, and the C content is adjusted according to the needs, generally 0.1-0.4%. When making tools, C can reach 0.8-1.0%. Some of them add Mo, V, Nb, etc. to improve the tempering stability. After heating at high temperature and cooling at a certain speed, the structure is basically martensite. According to the difference between C and alloy elements, some of them may contain a small amount of ferrite, retained austenite or alloy carbide. Phase transformation occurs during heating and cooling. Therefore, the microstructure and morphology can be adjusted in a wide range to change the properties. The corrosion resistance is not as good as that of Austenitic, Ferritic and duplex stainless steel. It has better corrosion resistance in organic acid and worse in sulfuric acid, hydrochloric acid and other media.
Generally, the content of Cr is 17-30% and the content of Ni is 3-13%. In addition, Mo, Cu, Nb, N, W and other alloy elements are added, and the content of C is very low. According to the different proportion of alloy elements, some are mainly ferrite, some are mainly austenite, forming two-phase stainless steel with two phases at the same time. Because it contains ferrite and strengthening elements, after heat treatment, its strength is slightly higher than that of austenitic stainless steel, and its plasticity and toughness are better, so it is basically impossible to adjust its properties by means of heat treatment. It has high corrosion resistance, especially in CL medium and sea water. It has good resistance to pitting corrosion, crevice corrosion and stress corrosion.
Precipitation hardening stainless steel
The composition is characterized by not only C, Cr, Ni and other elements, but also Cu, Al, Ti and other elements that can age precipitate. The mechanical properties can be adjusted by heat treatment, but the strengthening mechanism is different from that of martensitic stainless steel. Due to its strengthening by precipitation, C can be controlled very low, so its corrosion resistance is better than that of martensitic stainless steel, which is equivalent to Cr Ni austenitic stainless steel.
The composition characteristics of stainless steel, which is mainly composed of Cr alloy elements, are the basic conditions for stainless steel to be rust and corrosion resistant. In order to give full play to the role of alloy elements and obtain ideal mechanical and corrosion resistance, it must be realized by heat treatment.
Heat treatment of ferritic stainless steel
In general, ferrite stainless steel is a stable single ferrite structure without phase transformation during heating and cooling. Therefore, the mechanical properties can not be adjusted by heat treatment. Its main purpose is to reduce brittleness and improve the resistance to intergranular corrosion.
① σ phase brittleness
Ferritic stainless steel is very easy to form σ phase, which is a CR rich metal compound, hard and brittle, especially easy to form in intergranular, making steel brittle and increasing intergranular corrosion sensitivity. The formation of σ phase is related to the composition. In addition to Cr, Si, Mn, Mo and so on all promote the formation of σ phase. In addition, they are related to the processing process, especially the heating and staying in the temperature range of 540-815 ℃ promote the formation of σ phase. However, the formation of σ phase is reversible, and re heating to a temperature higher than that of σ phase will re dissolve in the solid solution.
② 475 ℃ brittleness
When the ferritic stainless steel is heated for a long time in the range of 400 ~ 500 ℃, it will show the characteristics of increasing strength, decreasing toughness and increasing brittleness, especially at 475 ℃, which is called 475 ℃ brittleness. This is because, at this temperature, the CR atoms in the ferrite will be rearranged to form a small CR rich region, which is coherent with the parent phase, causing lattice distortion, generating internal stress, increasing the hardness and brittleness of the steel. At the same time of the formation of rich CR zone, there must be poor CR zone, which has a negative impact on the corrosion resistance. When the steel is reheated above 700 ℃, the distortion and internal stress will be eliminated, and the brittleness will disappear at 475 ℃.
③ High temperature brittleness
When heated to above 925 ℃ and cooled down rapidly, Cr, C, N and other compounds precipitated in the intragranular and grain boundary, resulting in the increase of brittleness and intergranular corrosion. The compound can be eliminated by rapid cooling after heating at 750 ~ 850 ℃.
In order to eliminate σ phase, 475 ℃ brittleness and high temperature brittleness, annealing treatment can be used, heating at 780 ~ 830 ℃, heat preservation, and then air cooling or furnace cooling.
For ultra pure ferritic stainless steel (C ≤ 0.01%, strictly control Si, Mn, S, P), the annealing temperature can be increased.
② Stress relief treatment
After welding and cold working, parts may produce stress. If annealing treatment is not suitable for specific conditions, heating, heat preservation and air cooling can be carried out in the range of 230~370 ℃, which can eliminate part of internal stress and improve plasticity.
Heat treatment of austenitic stainless steel
The effect of Cr, Ni and other alloy elements in austenitic stainless steel makes MS point lower than room temperature (- 30 to – 70 ℃). Therefore, no phase transformation occurs above room temperature during heating and cooling. Therefore, the main purpose of heat treatment of austenitic stainless steel is not to change the mechanical properties, but to improve the corrosion resistance.
Solid solution treatment of austenitic stainless steel
① Precipitation and dissolution of alloy carbide in steel
C is one of the alloy elements in steel. Apart from strengthening a little, it is not good for corrosion resistance. Especially when C and Cr form carbide, the effect is worse, so we should try to reduce its existence. Therefore, the solubility of C in austenite is higher at high temperature and lower at low temperature. It has been reported that the solubility of C in austenite is 0.34% at 1200 ℃, 0.18% at 1000 ℃, 0.02% at 600 ℃, and even less at room temperature. Therefore, when the steel is heated to a high temperature, the C-Cr compound will be fully dissolved, and then it will be cooled quickly, so that it will not be precipitated, so as to ensure the corrosion resistance of the steel, especially the intergranular corrosion resistance.
② Sigma phase
If austenitic steel is heated for a long time in the range of 500-900 ℃, or Ti, Nb, Mo and other elements are added to the steel, it will promote the precipitation of σ phase, increase the brittleness and reduce the corrosion resistance of the steel. The way to eliminate σ phase is to dissolve it at a temperature higher than its possible precipitation temperature, and then cool it quickly to prevent the precipitation.
In gb1200 standard, the recommended heating temperature range is wide: 1000 ~ 1150 ℃, usually 1020-1080 ℃. Consider the specific brand composition, casting or forging, and adjust the heating temperature appropriately within the allowable range. When the heating temperature is low, C-Cr carbides can not be fully dissolved, and the temperature is too high, there are also problems of grain growth and corrosion resistance reduction.
Cooling mode: it should be cooled at a faster speed to prevent carbide from re precipitation. In China and some other national standards, “fast cooling” after solid solution is indicated. Based on different literature and practical experience, the scale of “fast” can be grasped as follows:
If the content of C ≥ 0.08%, the content of Cr > 22%, and the content of Ni is high, and if the content of C < 0.08%, but the effective size > 3mm, the water should be cooled;
- C content < 0.08%, size < 3mm, wind cooling;
- Air cooling is allowed if the effective size is less than or equal to 0.5mm.
Stabilization heat treatment of austenitic stainless steel
Stabilization heat treatment is limited to austenitic stainless steel containing stabilization elements Ti or Nb, such as 1Cr18Ni9Ti, 0cr18ni11nb, etc.
As mentioned before, the reason for the decrease of corrosion resistance of austenitic stainless steel is that CR combines with C to synthesize Cr23C6 compound and precipitates at the grain boundary. Cr is a strong carbide forming element. As long as there is a chance, it combines with C and precipitates. Therefore, the steel is filled with elements Ti and Nb with stronger affinity than Cr and C, and conditions are created to make C preferentially combine with Ti and Nb, reduce the chance of combination of C and Cr, and keep Cr in austenite stably, so as to ensure the corrosion resistance of steel. Stabilizing heat treatment plays the role of combining Ti, Nb and C to stabilize Cr in austenite.
Heating temperature: this temperature should be higher than the dissolution temperature of Cr23C6 (400-825 ℃), lower than or slightly higher than the initial dissolution temperature of tic or NBC (for example, the dissolution temperature range of tic is 750-1120 ℃), and the stabilization heating temperature is generally selected at 850-930 ℃, which will make Cr23C6 fully dissolved, make Ti or Nb combine with C, while CR will remain in austenite.
Cooling mode: air cooling is generally adopted, water cooling or furnace cooling can also be adopted, which shall be determined according to the specific conditions of parts. The cooling rate has no great influence on the stabilization effect. According to our experimental results, the cooling rate is 0.9 ℃ / min and 15.6 ℃ / min when cooling from 900 ℃ to 200 ℃, compared with that of metallographic structure, hardness and intergranular corrosion resistance.
Stress relief treatment of austenitic stainless steel
For parts made of austenitic stainless steel, there are inevitable stresses, such as processing stress and welding stress during cold working. The existence of these stresses will bring adverse effects, such as: the impact on dimensional stability; when the parts with stress are used in medium containing Cl, H2S, NaOH, etc., stress corrosion cracking will occur, which is a kind of local, unexpected damage, and is very harmful. Therefore, the austenitic stainless steel used in some working conditions should minimize the stress, which can be achieved by the method of stress relief.
If the conditions permit, the solution treatment and stabilization treatment can eliminate the stress better (the solution water cooling can also produce certain stress), but sometimes this method is not allowed, such as the pipe fittings in the circuit, the finished parts without allowance, the parts with special complex shape and easy deformation, etc., at this time, the stress relief method of heating below 450 ℃ can be used, or the stress relief method can be used Except partial stress. If the work piece is used in the environment of strong stress corrosion, the stress must be eliminated completely. When selecting materials, consideration should be given, such as the use of steel with stable elements, or the use of ultra-low carbon austenitic stainless steel.
Heat treatment of martensitic stainless steel
Compared with ferritic stainless steel, austenitic stainless steel and duplex stainless steel, the most outstanding feature of martensitic stainless steel is that the mechanical properties can be adjusted in a wide range through heat treatment to meet the needs of different service conditions. Different heat treatment methods have different effects on corrosion resistance.
Microstructure of martensitic stainless steel after quenching
According to the chemical composition:
- 0Cr13, 1Cr13 and 1Cr17Ni2 are martensite + a little ferrite;
- 2Cr13, 3Cr13 and 2cr17ni2 are basically martensitic;
- 4Cr13 and 9Cr18 are alloy carbide on martensitic matrix;
- 0Cr13Ni4Mo and 0cr13ni6mo are martensitic matrix with retained austenite.
Corrosion resistance and heat treatment of martensitic stainless steel
Heat treatment of martensitic stainless steel can not only change mechanical properties, but also have different effects on corrosion resistance. Take tempering after quenching as an example: after quenching into martensite, low temperature tempering is adopted, which has high corrosion resistance; medium temperature tempering at 400-550 ℃, which has low corrosion resistance; high temperature tempering at 600-750 ℃, which has improved corrosion resistance.
Heat treatment process and function of martensitic stainless steel
Different annealing methods can be adopted according to different purposes and functions to be achieved:
- It is only required to reduce hardness, facilitate processing and eliminate stress. Low temperature annealing (some also called incomplete annealing) can be used for heating at 740 ~ 780 ℃, and air cooling or furnace cooling hardness can guarantee 180 ~ 230hb;
- In order to improve the structure of forging or casting, lower hardness and ensure the direct application of low performance, full annealing can be used to heat 870 ~ 900 ℃, heat preservation and furnace cooling, or cooling to below 600 ℃ at the speed of ≤ 40 ℃ / h. The hardness can reach 150-180hb;
- Isothermal annealing can replace complete annealing and achieve the purpose of complete annealing. The heating temperature is 870 ~ 900 ℃, the furnace is cooled to 700 ~ 740 ℃ after heating and heat preservation (refer to the transformation curve), and the furnace is cooled to below 550 ℃ for a long time (refer to the transformation curve). The hardness can reach 150-180hb. This kind of isothermal annealing is also an effective way to improve the bad structure after forging, improve the mechanical properties after quenching and tempering, especially the impact toughness.
The main purpose of martensitic stainless steel quenching is to strengthen. When the steel is heated above the critical temperature and kept warm, the carbide is fully dissolved in austenite, and then cooled at a proper cooling rate to obtain the quenched martensite structure.
- Selection of heating temperature: the basic principle is to ensure the formation of austenite, and make alloy carbide fully dissolved in austenite, homogenization; also can not make austenite grain coarse or there is ferrite or retained austenite in the structure after quenching. This requires that the quenching temperature should not be too low or too high. For martensitic stainless steel, the range of temperature introduced and recommended by different materials is slightly different, and the temperature range is wide. According to our experience, heating in the temperature range of 980 ~ 1020 ℃ is usually enough. Of course, for special steel grade, special composition control or special requirements, the heating temperature should be appropriately reduced or increased, but the heating principle should not be violated.
- Cooling mode: due to the composition characteristics of martensitic stainless steel, the austenite is relatively stable, the C curve moves to the right, and the critical cooling speed is relatively small, so the effect of quenching martensite can be obtained by oil cooling and air cooling. However, oil cooling should be used for the parts with high hardenability and mechanical properties, especially for the parts with high impact toughness.
After quenching, martensitic structure of martensitic stainless steel is obtained, which has high hardness, high brittleness and high internal stress, so it must be tempered. Martensitic stainless steel is basically used at two tempering temperatures:
- Tempering between 180 ~ 320 ℃. The tempered martensite structure is obtained, with high hardness and strength, but low plasticity and toughness, and good corrosion resistance. For example, tools, bearings and wear-resistant parts can be tempered at low temperature.
- The tempered sorbite structure was obtained by tempering between 600 ℃ and 750 ℃. It has a certain strength, hardness, plasticity, toughness and other good comprehensive mechanical properties. According to the different requirements of strength, plasticity and toughness, lower or upper temperature tempering can be used. The structure also has good corrosion resistance.
- In general, tempering at 400-600 ℃ is not used, because in this temperature range, carbide with high dispersion is precipitated from martensite, resulting in tempering brittleness and reducing corrosion resistance. However, springs, such as 3Cr13 and 4Cr13 steel springs, can be backfilled at this temperature, with HRC up to 40-45, having good elasticity.
The cooling method after tempering can generally be air cooling, but for steel grades with tempering brittleness tendency, such as 1Cr17Ni2, 2Cr13, 0Cr13Ni4Mo, etc., it is better to use oil cooling after tempering. In addition, it should be noted that tempering should be carried out in time after quenching, no more than 24 hours in summer and no more than 8 hours in winter. If tempering cannot be carried out according to the process temperature in time, measures should also be taken to prevent static cracks.
Heat treatment of Ferritic Austenitic duplex stainless steel
Duplex stainless steel is a young member of stainless steel family, which develops late, but its characteristics are widely recognized and valued. The composition characteristics (high Cr, low Ni, Mo, n) and structure characteristics of duplex stainless steel make it have higher strength and plasticity than austenitic stainless steel and ferritic stainless steel; equivalent to the corrosion resistance of austenitic stainless steel; higher resistance to pitting corrosion, crevice corrosion and stress corrosion damage than any stainless steel in CL medium and sea water.
Elimination of secondary austenite
At higher temperature (such as casting or forging), the amount of ferrite increases. When the temperature is over 1300 ℃, it can be a single-phase ferrite. This kind of high-temperature ferrite is unstable. After aging at a lower temperature, there will be austenite precipitation. This kind of austenite is called secondary austenite. The amount of Cr and N in this kind of austenite is less than that of normal austenite, so it may become a corrosion source, so it should be eliminated by heat treatment.
Elimination of Cr23C6 carbide
Cr23C6 precipitated from dual phase steel below 950 ℃ will increase brittleness and decrease corrosion resistance, which should be eliminated.
③ Elimination of nitride Cr2N, CrN
Because of the N element in the steel, nitride can be formed with Cr, which affects the mechanical and corrosion resistance.
Elimination of intermetallic phase
The composition characteristics of dual phase steel can promote the formation of some intermetallic phases, such as σ phase and γ phase, which reduce corrosion resistance and increase brittleness, should be eliminated.
Similar to austenitic steel, it adopts solid solution treatment, heating temperature is 980 ~ 1100 ℃, then rapid cooling, generally water cooling.
Heat treatment of precipitation hardening stainless steel
Precipitation hardening stainless steel is a kind of stainless steel which has been tested, summarized and innovated in human practice. Among the early stainless steels, ferritic stainless steel and austenitic stainless steel have better corrosion resistance, but their mechanical properties cannot be adjusted by heat treatment, which limits their functions. However, martensitic stainless steel can be heat treated to adjust its mechanical properties in a wide range, but its corrosion resistance is poor.
It has lower C content (generally ≤ 0.09%), higher Cr content (generally ≥ 14%), and other elements such as Mo and Cu, which makes it have higher corrosion resistance, even equivalent to austenitic stainless steel. Through solution and aging treatment, the structure of precipitation hardening phase on martensite matrix can be obtained, so it has higher strength, and the strength, plasticity and toughness can be adjusted in a certain range according to the adjustment of aging temperature. In addition, the heat treatment method of solution first and precipitation strengthening according to the precipitation phase can be processed under the condition of low hardness after solution treatment, and then aging strengthening can reduce the processing cost, which is superior to martensitic steel.
Martensitic precipitation hardening stainless steel and its heat treatment
The characteristics of martensitic precipitation hardening stainless steel are: the starting temperature ms of austenite to martensite transformation is above room temperature. After austenitizing by heating and cooling at a faster speed, the lath like martensite matrix is obtained. After aging, fine particles of Cu are precipitated from the lath martensite matrix and strengthened.
Example: in GB1220 standard, the typical brand is 0Cr17Ni4Cu4Nb (ph17-4)
The composition (%) is as follows: C ≤ 0.07, Ni: 3 ~ 5, Cr: 15.5 ~ 17.5, Cu: 3 ~ 5, Nb: 0.15 ~ 0.45; MS point is about 120 ℃; MZ point is about 30 ℃.
Heating temperature is 1020-1060 ℃, after heat preservation, water cooling or oil cooling, the structure is lath martensite, hardness is about 320hb. The heating temperature should not be too high. If it is higher than 1100 ℃, the ferrite content, Ms point, retained austenite and hardness of the structure will increase, and the effect of heat treatment will not be good.
According to the different aging temperature, the dispersion and particle size of precipitates are different, but they have different mechanical properties.
According to GB1220, properties after aging at different aging temperatures
1040℃ Solid solution
Heat treatment of semi austenitic stainless steel
The MS point of the steel is generally slightly lower than the room temperature, so after the solid solution treatment is cooled to the room temperature, the austenite structure is obtained, and the strength is very low. In order to improve the strength and hardness of the matrix, it needs to be reheated to 750-950 ℃ for heat preservation. At this stage, carbide will be precipitated in the austenite, the austenite stability will be reduced, and the MS point will be raised above the room temperature, and the martensite structure will be obtained when the steel is cooled again. In some cases, cold treatment (below zero treatment) can be added, and then the steel can be reaged to obtain the strengthening steel with precipitates on the martensitic matrix.
For example, in gb1220 standard, the recommended brand of precipitated stainless steel is 0cr17ni7al (ph17-7)
- Composition (%): C ≤ 0.09, Cu ≤ 0.5, Ni: 6.5 ~ 7.5, Cr: 16 ~ 18, al: 0.75 ~ 1.5;
Solution + adjustment + aging treatment
- At 1040 ℃, austenite can be obtained by water cooling or oil cooling after heating and heat preservation, and the hardness is about 150HB;
- After adjusting the temperature to 760 ℃, the alloy carbide in austenite is precipitated and the stability of austenite is reduced. The MS point is increased to about 50-90 ℃, and the lath martensite is obtained after cooling. At this time, the hardness is about 290hb;
- After aging at 560 ℃, Al and compound precipitated, the steel was strengthened, and the hardness was about 340hb.
Solution + adjustment + cold treatment + aging
- After solution treatment, austenite structure was obtained by heating at 1040 ℃ and water cooling;
- At 955 ℃, Ms point was increased and lath martensite was obtained after cooling;
- After cold treatment at – 73 ℃ for 8h, the retained austenite was reduced and the maximum martensite was obtained;
- The aging treatment temperature is 510-560 ℃, which makes al precipitate. After strengthening treatment, the hardness can reach 336HB.
Solution + cold deformation + aging
- The austenite structure was obtained by solution treatment at 1040 ℃ and water cooling;
- In cold deformation, austenite is transformed into martensite at MD point by using the principle of cold working deformation strengthening, and the amount of cold working deformation is more than 30-50%;
- Aging treatment: heat aging at about 490 ℃ to make al precipitate and harden.
- It has been reported that the hardness of solid solution austenite is 430hb and σ B is 1372n / mm2 after 57% cold rolling. After aging at 490 ℃, the hardness is 485hb and σ B is 1850n / mm2.
It can be seen that the mechanical properties of precipitation hardening martensitic stainless steel can reach the properties of martensitic stainless steel after proper treatment, while the corrosion resistance is equivalent to that of austenitic stainless steel. It should be pointed out that martensitic stainless steel and precipitation hardening stainless steel can be strengthened by heat treatment, but the strengthening mechanism is different. Due to the characteristics of precipitation hardening stainless steel, it has been widely used.
Source: China Stainless Steel 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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Can stainless steel be heat treated?
Austenitic stainless steels cannot harden via heat treatment. Instead, these steels work harden (they attain hardness during their manufacture and formation). Annealing these stainless steels softens them, adds ductility and imparts improved corrosion resistance.
How do you heat treat 304 stainless steel?
Stainless steel 304 cannot be hardened by heat treatment. Solution treatment or annealing can be done by rapid cooling after heating to 1010-1120°C.
How do you heat treat 316l stainless steel?
For the stainless steel 316 alloy the solution anneal is accomplished by heating in the 1900 to 2150° F (1040 to 1175° C) temperature range followed by air cooling or a water quench, depending on section thickness.
How do you heat treat 321 and 347 stainless steel?
The annealing temperature range for Alloys 321 and 347 is 1800 to 2000°F (928 to 1093°C). While the primary purpose of annealing is to obtain softness and high ductility, these steel may also be stress relief annealed within the carbide precipitation range 800 to 1500°F (427 to 816°C), without any danger of subsequent intergranular corrosion. Relieving strains by annealing for only a few hours in the 800 to 1500°F (427 to 816°C) range will not cause any noticeable lowering in the general corrosion resistance, although prolonged heating within this range does tend to lower the general corrosion resistance to some extent. As emphasized, however, annealing in the 800 to 1500°F (427 to 816°C) temperature range does not result in a susceptibility to intergranular attack.
For maximum ductility, the higher annealing range of 1800 to 2000°F (928 to 1093°C) is recommended.
When fabricating chromium-nickel stainless steel pipe into equipment requiring the maximum protection against carbide precipitation obtainable through use of a stabilized grade, it is essential to recognize that there is a difference between the stabilizing ability of columbium and titanium. For these reasons, the degree of stabilization and of resulting protection may be less pronounced when Alloy 321 is employed.
When maximum corrosion resistance is called for, it may be necessary with Alloy 321 to employ a corrective remedy which is known as a stabilizing anneal. It consists of heating to 1550 to 1650°F (843 to 899°C) for up to five hours depending on thickness. This range is above that within which chromium carbides are formed and is sufficiently high to cause dissociation and solution of any that may have been previously developed. Furthermore, it is the temperature at which titanium combines with carbon to form harmless titanium carbides. The result is that chromium is restored to solid solution and carbon is forced into combination with titanium as harmless carbides.
This additional treatment is required less often for the columbium-stabilized Alloy 347.
When heat treatment are done in an oxidizing atmosphere, the oxide should be removed after annealing in a descaling solution such as a mixture of nitric acid and hydrofluoric acids. These acids should be thoroughly rinsed off the surface after cleaning.
These alloys cannot be hardened by heat treatment.