Heat treatment of stainless steel

Heat treatment of stainless steel

Classification and main characteristics of stainless steel

Stainless steel has a variety of classification methods, such as chemical composition, functional characteristics, metallurgical organization and heat treatment characteristics. From heat treatment considerations, according to the metallurgical organization and heat treatment characteristics are more practical significance.

Stainless steel →	Ferritic stainless steel
	Austenitic stainless steel
	Martensitic stainless steel
	Ferritic-austenitic duplex stainless steel
	Precipitation hardening stainless steel

Ferritic stainless steel

The main alloying element is Cr, or add a small amount of stable ferrite elements, such as Al, Mo, etc., the organization is ferrite. Strength is not high, can not adjust the performance by heat treatment, there is a certain plasticity, brittleness is large. Good corrosion resistance in oxidizing media (such as nitric acid), poor corrosion resistance in reducing media.

Austenitic stainless steel

Contains high Cr, generally greater than 18%, and contains about 8% Ni, some to Mn instead of Ni, to further improve corrosion resistance, there have to add Mo, Cu, Si, Ti, Nb and other elements. Phase change does not occur when heated and cooled, can not be strengthened by heat treatment methods, with low strength, high plasticity and high toughness. It has strong corrosion resistance to oxidizing medium, and has better resistance to intergranular corrosion after adding Ti and Nb.

Martensitic stainless steel

Martensitic stainless steel mainly contains 12 ~ 18% Cr, and according to the need to adjust the amount of C, generally in 0.1 ~ 0.4%, for the production of tools, C up to 0.8 ~ 1.0%, some to improve the tempering stability, adding Mo, V, Nb, etc.. After high temperature heating and cooling at a certain rate, the organization is basically martensite, according to the differences in C and alloying elements, some may contain a small amount of ferrite, residual austenite or alloy carbide. Phase changes occur when heated and cooled, so the organization and morphology can be adjusted in a wide range, thus changing the properties. Corrosion resistance is not as good as austenitic, ferritic and duplex stainless steel, in organic acids have better corrosion resistance, in sulfuric acid, hydrochloric acid and other media corrosion resistance is poor.

Ferritic-austenitic duplex stainless steel

Generally contains Cr 17-30%, Ni content 3-13%, in addition to the addition of Mo, Cu, Nb, N, W and other alloying elements, the amount of C control is very low, according to the proportion of alloying elements are different, some are mainly ferrite, some are mainly austenite, constituting two identical when the presence of duplex stainless steel. Because it contains ferrite and strengthening elements, after heat treatment, slightly higher strength than austenitic stainless steel, good plasticity and toughness, basically can not adjust the performance by means of heat treatment. There is a high corrosion resistance, especially in the Cl- containing media, seawater, there is better resistance to pitting and crevice corrosion, stress corrosion characteristics.

Precipitation hardening stainless steel

The composition is characterized by the presence of elements such as C, Cr, Ni, etc., in addition to Cu, Al, Ti and other elements that can age precipitated precipitates. Mechanical properties can be adjusted by means of heat treatment, but its strengthening mechanism is different from that of martensitic stainless steel. Because it relies on precipitation phase strengthening, so C can be controlled very low, and thus its corrosion resistance is better than martensitic stainless steel, and Cr-Ni austenitic stainless steel equivalent.

Heat treatment of stainless steel

Stainless steel is characterized by a large number of alloying elements consisting mainly of Cr, which is the basic condition for its stainless and corrosion resistance. To give full play to the role of alloying elements and obtain the desired mechanical and corrosion resistance properties, it must also be achieved through heat treatment methods.

Heat treatment of ferritic stainless steel

Ferritic stainless steel is generally a stable single ferrite tissue heating, cooling does not occur phase change, so you can not adjust the mechanical properties of heat treatment methods, the main purpose is to reduce brittleness and improve the resistance to intergranular corrosion.
① σ-phase brittleness
Ferritic stainless steel is very easy to generate σ-phase, which is a Cr-rich metal compounds, hard and brittle, especially easy to form in the intergranular, making the steel brittle, and increase the susceptibility to intergranular corrosion. σ-phase formation and composition, in addition to Cr, Si, Mn, Mo, etc. are to promote the formation of σ-phase; also related to the processing, especially in the 540 ~ 815 ℃ zone heating, dwell, more promote the formation of σ-phase. But the σ-phase formation is reversible, reheating to a temperature higher than the σ-phase formation will be re-dissolved in the solid solution.
② Brittleness at 475℃
Ferritic stainless steel in the 400 ~ 500 ℃ zone for a long time to heat, will show the characteristics of increased strength, toughness, that is, increased brittleness, especially at 475 ℃ most obvious, said 475 ℃ brittleness. This is because, at this temperature, the Cr atoms in the ferrite will be rearranged to form a small Cr-rich region, with the parent phase of the common lattice, causing dot matrix distortion, resulting in internal stress, so that the steel hardness increases, brittleness increases. Cr-rich area formed at the same time, there must be poor Cr area, which has a negative impact on corrosion resistance. When the steel is reheated above 700 ℃ temperature, distortion, internal stress will be eliminated, 475 ℃ brittleness disappears.
③ High temperature brittleness
Heated to 925 ℃ or more, and with rapid cooling down, Cr, C, N and other compounds formed in the crystal, grain boundary precipitation, causing increased brittleness and the occurrence of intergranular corrosion. This compound can be in 750 ~ 850 ℃ temperature heating after fast cooling to be eliminated.
Heat treatment process.
① Annealing
To eliminate σ-phase, 475°C brittleness and high-temperature brittleness, annealing treatment can be applied by heating at 780~830°C, holding, and then air-cooling or furnace cooling.
For ultra-pure ferritic stainless steel (containing C ≤ 0.01%, strict control of Si, Mn, S, P), the annealing heating temperature can be increased some.
② Stress relief treatment
After welding and cold working, parts may produce stress, if the specific situation is not suitable for annealing treatment, can be in the range of 230 ~ 370 ℃ heating, holding, air cooling, can eliminate some of the internal stress, improve plasticity.

Heat treatment of austenitic stainless steel

The effect of Cr, Ni and other alloying elements in austenitic stainless steel results in the Ms point dropping below room temperature (-30 to -70°C). Ensure that the austenite organization is stable, so that when heated and cooled, no phase change occurs above room temperature. Therefore, the main purpose of heat treatment of austenitic stainless steel is not to change the mechanical properties, but to improve corrosion resistance.

Solid solution treatment of austenitic stainless steel

Process.
In the GB1200 standard, the recommended heating temperature range is wide: 1000 ~ 1150 ℃, usually using 1020-1080 ℃. Consider the specific grade composition, whether it is castings or forgings, etc., and adjust the heating temperature appropriately within the allowable range. Heating temperature is low, C-Cr carbide can not be fully dissolved, the temperature is too high, there is also grain growth, reducing corrosion resistance problems.
Cooling: It should be cooled at a faster rate to prevent the carbide from precipitating again. In China and some other national standards, marked solid solution after “fast cooling”, a combination of different literature and practical experience, “fast” scale can be mastered as follows.

• C ≥ 0.08%; Cr > 22%, Ni higher; C < 0.08%, but the effective size of > 3mm, should be water-cooled;
• C content <0.08%, size <3mm, air-coolable;
• Air-coolable for effective size ≤ 0.5mm.

Stabilization heat treatment of austenitic stainless steel

Stabilization heat treatment is limited to austenitic stainless steels containing stabilizing elements Ti or Nb, such as 1Cr18Ni9Ti, 0Cr18Ni11Nb, etc.
Process.
Heating temperature: This temperature should be higher than the dissolution temperature of Cr23C6 (400-825°C) and lower or slightly higher than the starting dissolution temperature of TiC or NbC (e.g., the dissolution temperature interval of TiC is 750-1120°C), and the stabilization heating temperature is generally chosen to be 850-930°C, which will allow Cr23C6 to dissolve sufficiently for Ti or Nb to combine with C in it again, while Cr remains in the austenite.
Cooling method: generally air-cooled, but also water-cooled or furnace-cooled, which should be determined according to the specific circumstances of the parts. The cooling rate has no significant effect on the stabilization effect. From the results of our experimental research, from the stabilization temperature of 900 ℃ cooling to 200 ℃, the cooling rate of 0.9 ℃ / min and 15.6 ℃ / min, compared to the metallographic organization, hardness, intergranular corrosion resistance is basically equivalent.

Stress relief treatment for austenitic stainless steel

Process.
In the conditions allowed, the use of solid solution treatment, stabilization treatment can be better stress relief (solid solution water cooling will also produce a certain amount of stress), but, sometimes this method is not allowed, such as the circuit of the fittings, no margin of completion of the parts, the shape of particularly complex easily deformable parts, etc., then the stress relief method can be used to heat the temperature below 450 ℃, can also eliminate part of the stress. If the workpiece is used in a strong stress corrosion environment, the stress must be completely eliminated, then in the selection of materials, should be considered, such as the use of steel containing stable elements, or the use of ultra-low carbon austenitic stainless steel.

Heat treatment of martensitic stainless steel

Martensitic stainless steel compared to ferritic stainless steel, austenitic stainless steel, duplex stainless steel is the most prominent feature is that the mechanical properties can be adjusted through heat treatment methods, in a wide range to meet the needs of different use conditions. Different heat treatment methods also have different effects on corrosion resistance.
① The tissue state of martensitic stainless steel after quenching
Depending on chemical composition

• 0Cr13, 1Cr13, 1Cr17Ni2 are martensite + small amount of ferrite;
• 2Cr13, 3Cr13, 2Cr17Ni2 are basically martensitic;
• 4Cr13, 9Cr18 are alloyed carbides on a martensitic matrix;
• 0Cr13Ni4Mo, 0Cr13Ni6Mo for martensite matrix with residual austenite.

② Corrosion resistance of martensitic stainless steel and heat treatment
Martensitic stainless steel heat treatment can not only change the mechanical properties, corrosion resistance also has a different role. After quenching and tempering, for example: after quenching into martensite, using low-temperature tempering, with high corrosion resistance; using 400-550 ℃ tempering, corrosion resistance is lower; using 600-750 ℃ high-temperature tempering, corrosion resistance and improved.
③ Martensitic stainless steel heat treatment process methods and role
Annealing
Different annealing methods can be used depending on the purpose and role to be achieved:
Only required to reduce hardness, easy processing, stress relief, can be used low-temperature annealing (some also called incomplete annealing) heating temperature optional 740 ~ 780 ℃, air cooling or furnace cooling hardness can be guaranteed 180 ~ 230HB.
Requirements to improve the forging or casting organization, lower hardness and ensure that not high performance direct application, can be used to fully anneal general heating 870 ~ 900 ℃, holding after furnace cooling, or cooling to ≤ 40 ℃ / h speed to 600 ℃ or less out of the furnace. Hardness up to 150~180HB.
Isothermal annealing, its can replace complete annealing, to achieve the purpose of complete annealing role. Heating temperature 870 ~ 900 ℃, heating and holding after furnace cooling to 700 ~ 740 ℃ (can refer to the transformation curve), a longer period of holding (refer to the transformation curve), and then furnace cooling to 550 ℃ or less out of the furnace. Hardness up to 150-180HB. this isothermal annealing, or improve the forging of poor organization, improve the quenching and tempering mechanical properties, especially the impact toughness of the effective way.
Quenching
The main purpose of martensitic stainless steel quenching is strengthening. The steel is heated above the critical point temperature, held so that the carbide is fully dissolved into the austenite, and then cooled at an appropriate cooling rate to obtain a quenched martensitic organization.
Heating temperature selection: the basic principle is to ensure the formation of austenite, and make the alloy carbide fully dissolved into the austenite, homogenization; also can not make the austenite grain coarse or quenched organization in the presence of ferrite or residual austenite. This requires quenching heating temperature can not be too low, nor too high. Martensitic stainless steel quenching heating temperature, different information introduced, the recommended range varies slightly, and, the temperature range is wide. According to our experience, generally selected in the 980 ~ 1020 ℃ range heating can be. Of course, for special steel grades, special composition control or special requirements, should be appropriate to reduce or increase the heating temperature, but not against the heating principle.
Cooling method: because of the martensitic stainless steel composition characteristics, so that the austenite is more stable, C curve right shift, the critical cooling rate is small, so with oil cooling, air cooling can obtain the effect of quenching martensite. But for the requirement of quenching depth, mechanical properties, especially high impact toughness of the parts, should be used oil cooling.
Tempering
Martensitic stainless steel quenching, martensitic organization, its high hardness, brittleness, internal stress, must be tempered. Martensitic stainless steel is basically used in two tempering temperatures.
Tempered between 180~320℃. Obtain tempered martensite organization, maintain high hardness, strength, but low plasticity, toughness, and good corrosion resistance. Such as tools, bearings, wear-resistant parts, etc. can be used low-temperature tempering.
Tempered between 600~750℃ to obtain tempered sowthite organization. Has a certain strength, hardness, plasticity, toughness and other good comprehensive mechanical properties, can be based on the strength, plasticity, toughness requirements of different degrees, the use of the lower or upper temperature tempering. This organization also has good corrosion resistance.
Tempering between 400 ~ 600 ℃ temperature, generally not used, because, in this temperature range tempering, from the martensite in the precipitation of high dispersion of carbide, resulting in tempering brittleness, reducing corrosion resistance. However, springs, such as 3Cr13, 4Cr13 steel springs, can be tempered at this temperature, HRC up to 40 ~ 45, with good elasticity.
Cooling after tempering, generally can be air-cooled, but for steel grades with a tendency to temper brittleness, such as 1Cr17Ni2, 2Cr13, 0Cr13Ni4Mo, etc., it is best to use tempering after oil cooling. In addition, it should be noted that the problem is that the quenching needs to be tempered in time, not more than 24hours in summer, not more8 than hours in winter, such as can not be tempered in time according to the process temperature, should also take measures to prevent the generation of static cracking.

Heat treatment of ferritic-austenitic duplex stainless steel

Duplex stainless steel is a young member of the stainless steel family, late in its development, but its characteristics are widely recognized and valued. Duplex stainless steel composition characteristics (high Cr, low Ni, plus Mo, N) and organizational characteristics, so that it has a higher strength, plasticity than austenitic stainless steel and ferritic stainless steel; equivalent to the corrosion resistance of austenitic stainless steel; in cl-media, seawater than any stainless steel are higher resistance to pitting, resistance to crevice corrosion and resistance to stress corrosion damage.
Role.
① Elimination of secondary austenite
In higher temperature conditions (such as casting or forging), the amount of ferrite increased, at 1300 ℃ or more, can become a single-phase ferrite, this high temperature ferrite is unstable, in the later lower temperature aging, there will be austenite precipitation, this austenite is called secondary austenite. This austenite in the amount of Cr, N less than normal austenite, so it may become a source of corrosion, so it should be eliminated through heat treatment.
② Elimination of CrC 236type carbide
Duplex steel below 950 ℃ will precipitate Cr23C6 increase brittleness, reduce corrosion resistance, should be eliminated.
③ Elimination of nitride CrN2, CrN
Because there is N element in the steel, can generate nitride with Cr, affecting the mechanics and corrosion resistance, should be eliminated.
④ Elimination of intermetallic phases
The composition characteristics of duplex steel will promote the formation of some intermetallic phases, such as σ-phase and γ-phase, which reduce corrosion resistance and increase brittleness and should be eliminated.
Process.
Similar to austenitic steel, solid solution treatment is used, with heating temperature of 980~1100℃, followed by fast cooling, usually with water cooling.

Precipitation hardening stainless steel heat treatment

Precipitation hardening stainless steel is relatively late in the development of human practice after experimentation, summary, innovation of stainless steel species. The first appearance of stainless steel, ferritic stainless steel, austenitic stainless steel has better corrosion resistance, but can not be adjusted by heat treatment methods of mechanical properties, limiting its role. And martensitic stainless steel can use heat treatment methods to adjust the mechanical properties in a larger range, but the corrosion resistance is poor.
Features
It has a low amount of C (generally ≤ 0.09%), a high amount of Cr (generally ≥ 14% above), plus Mo, Cu and other elements, which makes it a high corrosion resistance, even with austenitic stainless steel equivalent. Through solid solution and aging treatment, you can obtain the organization of precipitation hardening phase precipitated on the martensite matrix, thus having higher strength, and can be adjusted according to the aging temperature, within a certain range of strength, plasticity, toughness. In addition, first solid solution, and then according to the precipitation phase precipitation strengthening heat treatment, can be processed after solid solution treatment, the hardness is lower in the case of basic molding, and then by aging strengthening, reducing processing costs, better than martensitic steel.

Classification.

Precipitation hardening stainless steel →	Martensite type →	common
	Semi Austrian type →	common
	Austrian type
	Austenite + ferrite type

① Martensitic precipitation hardening stainless steel and its heat treatment

Martensitic precipitation-hardening stainless steel is characterized by the beginning of the transformation from austenite to martensite at a temperature Ms above room temperature. After heating austenitization and cooling at a faster rate, a slate-like martensitic matrix is obtained, which is strengthened by precipitation of fine dots of Cu from the slate-martensitic matrix after aging.

• Example: In GB1220 standard, the typical grade 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℃.

Solid solution treatment.
The heating temperature is 1020-1060℃, water-cooled or oil-cooled after holding, the organization is slate-like martensite, hardness is about 320HB. Heating temperature should not be too high, if greater than 1100 ℃, will make the organization of the amount of ferrite increased, Ms point down, increased residual austenite, hardness decreased, the heat treatment effect is not good.
Time-sensitive processing.
Depending on the aging temperature, the dispersion and particle size of the precipitated precipitates are different, and there are different mechanical properties.

GB1220 standard stipulates that the performance after aging at different aging temperatures

② Semi-austenitic stainless steel heat treatment
This steel Ms point is generally slightly lower than room temperature, so the solid solution treatment after cooling to room temperature, get austenite organization, strength is very low, in order to improve the matrix strength, hardness, need to be heated again to 750-950 ℃, insulation, this stage, austenite will precipitate carbide, austenite stability is reduced, Ms point increased to room temperature above, and then cooled, get martensite organization. Some can also add cold treatment (sub-zero treatment), after which, then aging so that the steel eventually obtains a martensitic matrix with precipitated precipitates on the reinforced steel.

• Example: In the GB1220 standard, the recommended grade of this 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.

Solid solution + adjustment + aging treatment

• Solidification heating temperature 1040 ℃, heating and holding after water cooling or oil cooling to obtain austenite, hardness of about 150HB;
• Adjust the treatment temperature to 760 ℃, holding after air-cooling, so that the austenite alloy carbide precipitation, reduce austenite stability, improve Ms point to 50-90 ℃ or so, after cooling to obtain slatted martensite, the hardness can be about 290 HB;
• Then by 560 ℃ aging, Al and compounds precipitation precipitation, steel strengthening, hardness up to about 340 HB.

Solid solution + adjustment + cold treatment + aging

• Solid solution treatment is heated to 1040°C and water-cooled to obtain austenite structure;
• Adjust the treatment temperature 955°C to increase the Ms point and obtain slatted martensite after cooling;
• Cold treatment -73℃×8h to reduce the residual austenite in the tissue and to obtain the maximum martensite;
• Ageing treatment temperature of 510-560 ℃, so that Al precipitation, after strengthening treatment, the hardness can reach 336HB.

Solid solution + cold deformation + aging

• Solid solution treatment temperature is 1040°C, water cooling, to obtain austenite organization;
• Cold deformation, the use of cold working deformation strengthening principle, so that the austenite in the Md point into martensite, this cold working deformation should be more than 30-50%;
• Ageing treatment: heating and aging at about 490°C to precipitate and harden Al.

A report shows that solid solution austenite by 57% cold rolling deformation, hardness up to 430HB, σb up to 1372 N/mm², and then by 490 ℃ aging, hardness up to 485HB, σb up to 1850 N/mm².

As can be seen, precipitation hardening martensitic stainless steel after the correct treatment, mechanical properties can fully achieve martensitic stainless steel properties, while corrosion resistance is comparable to austenitic stainless steel. It should be noted here that martensitic stainless steel and precipitation hardening stainless steel, although both can be strengthened by heat treatment methods, but the strengthening mechanism is different. Due to the characteristics of precipitation-hardening stainless steel, so that it is valued and widely used.

Source: China Pipe Fittings Manufacturer – Yaang Pipe Industry (www.pipelinedubai.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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