Classification and main characteristics of stainless steel
There are many classification methods for stainless steel, such as chemical composition, functional characteristics, metallographic structure and heat treatment characteristics.
From the perspective of heat treatment, it is more practical to divide it according to the metallographic structure and heat treatment characteristics.
1. Ferritic stainless steel
The main alloying element is Cr, or to add a small amount of stable ferrite elements, such as Al, Mo, etc., and the structure is ferrite.
Strength is not high, which can not use heat treatment methods to adjust the performance, there is a certain plasticity and large brittleness.
It has good corrosion resistance in oxidizing media (such as nitric acid) and poor corrosion resistance in reducing media.
2. Austenitic stainless steel
It contains a high concentration of Cr, generally greater than 18% and about 8% Ni.
Some using Mn to replace Ni, which is to further improve corrosion resistance, and some have to add Mo, Cu, Si, Ti or Nb and other elements.
Heating and cooling do not occur when the phase change, which can not use heat treatment methods to strengthen, and it has the advantages of low strength, high plasticity and high toughness.
It has strong corrosion resistance to oxidizing media, and has good resistance to intergranular corrosion after adding Ti and Nb.
3. Martensitic stainless steel
Martensitic stainless steel mainly contains 12~18% Cr, and the amount of C can be adjusted according to needs, generally 0.1~0.4%.
For tools, C can reach 0.8~1.0%, and some are to improve the stability of tempering resistance by adding Mo, V,and Nb etc.
After heating at a high temperature and cooling at a certain speed, the structure is basically martensite.
Depending on the difference of C and alloying elements, some may contain a small amount of ferrite, retained austenite or alloy carbide.
Phase changes occur during heating and cooling.
Therefore, the structure and shape of the structure can be adjusted in a wide range, thereby changing the performance.
The corrosion resistance is not as good as that of austenitic, ferritic and duplex stainless steel.
It has better corrosion resistance in organic acids and poor corrosion resistance in sulfuric acid, hydrochloric acid and other media.
4. Austenoferritic steel stainless steel
Generally, the content of Cr is 17~30%, and the content of Ni is 3~13%.
In addition, alloying elements such as Mo, Cu, Nb, N and W are added, and the C content is controlled very low.
Depending on the proportion of alloying elements, some ferrite, some are mainly austenite, constituting two duplex stainless steels that exist simultaneously.
Because it contains ferrite and strengthening elements, after heat treatment, the strength is slightly higher than that of austenitic stainless steel and the plasticity and toughness are better, which is impossible to adjust the performance by heat treatment.
It has high corrosion resistance, especially in Cl- containing media and seawater, which has good resistance to pitting, crevice corrosion and stress corrosion.
5. Precipitation hardening stainless steel
The composition is characterized by the presence of C, Cr, Ni and other elements, but also contains Cu, Al and Ti etc. that can age precipitate precipitates.
Mechanical properties can be adjusted by means of heat treatment, but its strengthening mechanism is different from martensitic stainless steel.
Because of its reliance on precipitation phase strengthening, so C can be controlled very low, thus its corrosion resistance is better than martensitic stainless steel, and Cr-Ni austenitic stainless steel is equivalent.
Heat Treatment of Stainless Steel
Stainless steel is characterized by the composition of a large number of alloy elements mainly composed of Cr, which is the basic condition of stainless steel and corrosion resistance.
In order to fully use alloying elements, to obtain the ideal mechanical and corrosion resistance, it must also be realized through the heat treatment method.
1. Heat treatment of ferritic stainless steel
Ferritic stainless steel under normal circumstances is a stable single ferrite tissue heating, cooling does not occur phase change, so it can not use heat treatment to adjust the mechanical properties.
The main purpose is to reduce brittleness and improve resistance to intergranular corrosion.
① σ phase brittleness
Ferritic stainless steel is very easy to generate σ phase, which is a kind of Cr-rich metal compound with hard and brittle characteristics.
It is especially easy to form between crystals to make steel brittle and increase the sensitivity of intergranular corrosion.
The formation of σ phase is related to the composition, except Cr, Si, Mn and Mo, etc. all promote the formation of σ phase;
It is also related to the processing process, especially heating and staying in the range of 540~815℃, which promotes the formation of σ phase.
However, the formation of σ phase is reversible, and reheating above the formation temperature of σ phase will re-dissolve in a solid solution.
② Brittleness at 475℃
Ferritic stainless steel heated for a long time in the range of 400~500℃ will show the characteristics of increased strength, decreased toughness and increased brittleness, especially at 475℃, which is called 475℃ brittleness.
This is because, at this temperature, the Cr atoms in the ferrite will rearrange to form a small Cr-rich region, which is coherent with the parent phase, causing lattice distortion, generating internal stress, and increasing the hardness and brittleness of the steel.
When the Cr-rich area is formed, there must be a Cr-poor area, which has an adverse effect on the corrosion resistance.
When the steel is reheated to a temperature higher than 700°C, the distortion and internal stress will be eliminated, and the brittleness at 475°C will disappear.
③ High-temperature brittleness
When heating to above 925°C and rapidly cooling down, compounds such as Cr, C, and N will precipitate in the grains and grain boundaries, which causes increased brittleness and intergranular corrosion.
This compound can be eliminated by heating at a temperature of 750~850℃ and then rapidly cooling.
- In order to eliminateσ phase, brittleness at 475°C and brittleness at high temperature, annealing treatment can be used. It needs to heat and hold at 780~830°C, and then to use air cooling or furnace cooling.
- For ultra-pure ferritic stainless steel (C≤0.01%, strictly control Si, Mn, SandP), the annealing heating temperature can be increased.
② Stress-relieve treatment
After welding and cold-working, parts may have stress.
If annealing treatment is not suitable for specific circumstances, heating, heat preservation, and air cooling can be used in the range of 230~370℃, which can eliminate some internal stress and improve plasticity.
2. Heat treatment of austenitic stainless steel
Cr, Ni and other alloying elements in austenitic stainless steel result in the Ms point down to below room temperature (-30 to -70 ℃).
To ensure the stability of the austenitic organization, so that when heating and cooling, the phase change does not occur above room temperature.
Therefore, the main purpose of austenitic stainless steel heat treatment is not to change the mechanical properties, but to improve corrosion resistance.
① Solution treatment of austenitic stainless steel
① Precipitation and dissolution of alloy carbides in steel
C is one of the alloying elements contained in the steel.
In addition to having a little strengthening effect, it is detrimental to corrosion resistance, especially when C and Cr form carbides, the effect is even worse, so it should try to reduce its existence.
For this reason, according to the characteristic that C changes with temperature in austenite, that is, the solubility is large at high temperatures, and the solubility is small at low temperatures.
It is reported that the solubility of C in austenite is 0.34% at 1200℃;
It is 0.18% at 1000°C, 0.02% at 600°C, and even less at room temperature.
Therefore, the steel is heated to a high temperature to fully dissolve the C-Cr compound,.
Then, it is quickly cooled to stop precipitation, which is to ensure the corrosion resistance of the steel, especially the intergranular corrosion resistance.
If austenitic steel is heated in the range of 500-900°C for a long time, or when Ti, Nb, Mo and other elements are added to the steel, the σ-phase will be precipitated, which will increase the brittleness and reduce the corrosion resistance of the steel.
The means of eliminating the σ phase is also to dissolve it at a temperature higher than that at which it may precipitate, and then cool it rapidly to prevent re-precipitation.
In the GB1200 standard, the recommended heating temperature range: 1000 ~ 1150 ℃, usually 1020-1080 ℃.
Consider the specific grade composition, castings or forgings, etc., within the permissible range, the appropriate adjustment of the heating temperature.
Heating temperature is low, C-Cr carbide can not be fully dissolved, the temperature is too high, there is also the grain growth, reduce corrosion resistance problems.
Cooling method: should be cooled at a faster rate to prevent carbide precipitation again.
In China and some other national standards, it is indicated as “quick cooling” after solid solution, and the scale of “quick” can be grasped according to the following circumstances.
- Containing C amount ≥ 0.08%; Containing higher Cr amount > 22%, Ni amount; Containing C amount <0.08%, but the effective size > 3mm, should be water-cooled;
C content <0.08%, size <3mm, can be air-cooled;
The effective size ≤ 0.5mm can be air-cooled.
② 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 and 0Cr18Ni11Nb etc.
As mentioned earlier, Cr and C combine to form Cr23C6 type compounds and precipitate at the grain boundaries, which is the cause of the decline in the corrosion resistance of austenitic stainless steel.
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, which have a stronger affinity for Cr and C, and conditions are created to make C preferentially combine with Ti and Nb to reduce the chance of combining C with Cr.
The Cr is stably retained in the austenite, thus ensuring the corrosion resistance of the steel.
The stabilization 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 or slightly higher than the initial dissolution temperature of TiC or NbC (for example, the dissolution temperature range of TiC is 750-1120℃) to stabilize the heating temperature.
The stabilizing heating temperature is generally selected at 850-930℃, which will fully dissolve Cr23C6, allowing Ti or Nb to combine with C, while Cr will continue to remain in the austenite.
Cooling method: Air cooling is generally used while water cooling or furnace cooling can also be used, which should be determined according to the specific conditions of the parts.
The cooling rate has no major influence on the stabilization effect.
According to our experimental research results, when cooling from a stabilization temperature of 900°C to 200°C, the cooling rate is 0.9°C/min and 15.6°C/min, which are basically equivalent in metallographic structure, hardness, and intergranular corrosion resistance.
③ Austenitic stainless steel stress relief treatment
Parts made of austenitic stainless steel inevitably have stressed, such as processing stress and welding stress during cold-working.
The existence of these stresses will bring adverse effects:
the impact on dimensional stability;
stress corrosion cracking will occur when components with stress are used in Cl– media, H2S, NaOH and other media,
This is a kind of sudden damage that occurs locally and without warning, which is very harmful.
Therefore, the austenitic stainless steel parts used under certain working conditions should reduce the stress to the greatest extent, which can be accomplished by stress relief methods.
When conditions permit, the use of solution treatment and stabilization treatment can better eliminate stress (solid solution water cooling will also produce certain stress).
However, sometimes this method is not allowed, such as pipe fittings in the loop, finished workpieces with no margin, and easily deformable parts with particularly complex shapes.
At this time, the stress relief method of heating at a temperature below 450 ℃ can be used, and part of the stress can also be eliminated.
If the workpiece is used in a strong stress corrosion environment and the stress must be completely eliminated.
It should be considered when selecting materials, such as using steel with stable elements ultra-low carbon austenitic stainless steel.
3. Heat treatment of martensitic stainless steel
Compared with ferritic stainless steel, austenitic stainless steel and duplex stainless steel, the most prominent feature of martensitic stainless steel is that the mechanical properties can be adjusted in a wide range through heat treatment methods to meet the needs of different use conditions.
Different heat treatment methods also have different effects on corrosion resistance.
① The structure of martensitic stainless steel after quenching
Depending on the chemical composition
- 0Cr13, 1Cr13, 1Cr17Ni2 are martensite + a small amount of ferrite;
- 2Cr13, 3Cr13, 2Cr17Ni2 are basically martensitic structure;
- 4Cr13, 9Cr18 are alloy carbides on the martensite matrix;
- 0Cr13Ni4Mo and 0Cr13Ni6Mo have retained austenite on the martensite matrix.
② Corrosion resistance and heat treatment of martensitic stainless steel
Heat treatment of martensitic stainless steel can not only change the mechanical properties, but also have different effects on the corrosion resistance.
Take tempering after quenching as an example: after quenching into martensite, low-temperature tempering is used, which has high corrosion resistance;
Using 400-550℃ medium temperature tempering, the corrosion resistance is low;
adopting 600-750℃ high-temperature tempering, the corrosion resistance is improved.
③ The heat treatment process method and function of martensitic stainless steel
Depending on the purpose and effect to be achieved, different annealing methods can be used:
- It is only required to reduce the hardness, facilitate processing, and eliminate stress.
Low-temperature annealing (some also called incomplete annealing) can also be used.
The heating temperature can be 740~780℃, and the air cooling or furnace cooling hardness can be guaranteed 180~230HB;
- It is required to improve the forging or casting structure, lower the hardness and ensure the direct application of low performance, which can be used for complete annealing.
Generally, it is heated to 870~900℃, and the furnace is cooled after heat preservation, or cooled to below 600℃ at a rate of ≤40℃/h.
Hardness can reach 150~180HB;
- Isothermal annealing, which can replace complete annealing to achieve the purpose of complete annealing.
The heating temperature is 870~900℃.
After heating and holding, the furnace is cooled to 700~740℃ (refer to the transformation curve), and the furnace is kept for a longer time (refer to the transformation curve), and then the furnace is cooled to below 550°C.
The hardness can reach 150-180HB.
This isothermal annealing is also an effective way to improve the poor structure after forging and improve the mechanical properties after quenching and tempering, especially the impact toughness.
The main purpose of martensitic stainless steel quenching is strengthening.
The steel is heated to above the critical point temperature, kept warm to make the carbides fully dissolve into the austenite, and it is cooled at an appropriate cooling rate to obtain a quenched martensite structure.
- Heating temperature selection: The basic principle is to ensure the formation of austenite, and to fully dissolve alloy carbides into the austenite for homogenization;
It is not possible to make the austenite grains coarser or the presence of ferrite or retained austenite in the structure after quenching.
This requires that the quenching heating temperature cannot be too low or too high.
The martensitic stainless steel quenching heating temperature is slightly different from the introduction and recommended range of different materials, and the temperature range is wider.
According to our experience, it generally selects heating temperature in the range of 980~1020℃.
Of course, for special steel grades, special composition control, or special requirements, the heating temperature should be appropriately reduced or increased, but the heating principle should not be violated.
- Cooling method: Due to the compositional characteristics of martensitic stainless steel, the austenite is more stable, the C curve shifts to the right, and the critical cooling rate is lower.
Therefore, the effect of quenching martensite can be obtained by oil cooling or air cooling.
But for parts that require large hardening depth and mechanical properties, especially high impact toughness, oil cooling should be used.
After quenching the martensitic stainless steel, a martensitic structure with high hardness, high brittleness, and high internal stress is obtained, which must be tempered.
Martensitic stainless steel is basically used at two tempering temperatures:
- Tempering between 180~320℃can obtain tempered martensite structure, maintain high hardness and strength.
But it has low plasticity and toughness, and have good corrosion resistance.
For example, cutting tools, bearings and wear parts etc. can be tempered at low temperatures.
- Tempering between 600~750℃ to obtain tempered sorbite structure.
It has a certain degree of good comprehensive mechanical properties such as strength, hardness, plasticity and toughness.
Depending on the requirements for strength, plasticity and toughness, the lower or upper-temperature tempering can be used.
This structure also has good corrosion resistance.
- Tempering at a temperature between 400 and 600°C is generally not used because tempering in this temperature range will precipitate highly dispersed carbides from martensite, resulting in temper brittleness and reducing corrosion resistance.
However, springs, such as 3Cr13 and 4Cr13 steel springs, can be tempered at this temperature.
HRC can reach 40~45, which has good elasticity.
The cooling method after tempering can generally be air cooling, but for steel grades with a tendency to temper brittleness, such as 1Cr17Ni2, 2Cr13 and 0Cr13Ni4Mo etc., it is best to use oil cooling after tempering.
4. Heat Treatment of Ferritic-Austenitic Duplex Stainless Steel
Duplex stainless steel is a new member of the stainless steel family and developed late, but its characteristics have been widely recognized and valued.
The composition characteristics (high Cr, low Ni, plus Mo, N) and microstructure characteristics of duplex stainless steel make it have higher strength and plasticity than austenitic stainless steel and ferritic stainless steel;
It is equivalent to the corrosion resistance of austenitic stainless steel;
it has a higher resistance to pitting corrosion, crevice corrosion and stress corrosion damage than any stainless steel in cl-media and seawater.
① Eliminate secondary austenite
Under higher temperature conditions (such as casting or forging), the amount of ferrite increases,
Above 1300℃, it can become single-phase ferrite, which is unstable at high temperatures.
After aging at a lower temperature, austenite will be precipitated.
This austenite is called secondary austenite.
The amount of Cr and N in this austenite is less than normal austenite, so it may become a source of corrosion and it should be eliminated by heat treatment.
② Eliminate Cr23C6 carbide
Duplex steel will precipitate Cr23C6 below 950℃ to increase the brittleness and reduce corrosion resistance, which should be eliminated.
③ Eliminate nitrides Cr2N, CrN
Due to the N element in steel, it can form nitrides with Cr, which will affect the mechanical and corrosion resistance and should be eliminated.
④ Eliminate intermetallic phase
The compositional characteristics of dual-phase steel will promote the formation of some intermetallic phases, such as σ phase and γ phase, which reduce corrosion resistance and increase the brittleness and should be eliminated.
Similar to austenitic steel, it adopts solid solution treatment, the heating temperature is 980~1100℃, and then it is quickly cooled.
It generally uses water cooling.
5. Heat treatment of Precipitation hardening stainless steel
Precipitation hardening stainless steel is relatively late in development, and it is a kind of stainless steel that has been tested, summarized and innovated in human practice.
Among the stainless steels that appeared earlier, ferritic stainless steels and austenitic stainless steels have good corrosion resistance, but the mechanical properties cannot be adjusted by heat treatment methods, which limits their effects.
The martensitic stainless steel can be heat treated to adjust the mechanical properties in a larger range, but the corrosion resistance is poor.
It has a low C content (generally ≤0.09%), a higher Cr content (generally ≥14% or more), plus Mo, Cu and other elements, which makes it have higher corrosion resistance that is equivalent to Austenitic stainless steel.
Through solid solution and aging treatment, a structure with precipitation hardening phase precipitated on the martensite matrix can be obtained, so it has a higher strength.
The strength, plasticity and toughness can be adjusted within a certain range according to the adjustment of the aging temperature.
In addition, the heat treatment method of the solid solution followed by precipitation phase precipitation reinforcement can process basic shapes with low hardness after the solid solution treatment.
After re-strengthening by aging, it reduces processing costs and outperforming martensitic steels.
① Martensitic precipitation hardening stainless steel and its heat treatment
Martensitic precipitation-hardening stainless steel is characterized by an austenitic to martensitic transformation starting at a temperature (Ms) that above room temperature.
After heating austenitization and cooling at a faster rate, a slate-like martensitic matrix is obtained.
After aging, the fine mass of Cu precipitates from the slate-like martensitic matrix and is strengthened.
Example: In the 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:
When the heating temperature is 1020-1060 ℃, after heat preservation, water or oil cooling, the structure is lath martensite, the hardness is about 320HB.
The heating temperature should not be too high, if it is greater than 1100°C, the amount of ferrite in the structure will increase, the Ms point will decrease, the retained austenite will increase, the hardness will decrease, and the heat treatment effect will not be good.
Depending on the aging temperature, the dispersion and particle size of the precipitates are different, and they have different mechanical properties.
According to the GB1220 standard, the performance after aging at different aging temperatures:
② Heat treatment of semi-austenitic stainless steel
The Ms point of this steel is generally slightly lower than room temperature, so after the solution treatment is cooled to room temperature, an austenite structure is obtained with very low strength.
In order to improve the strength and hardness of the matrix, it needs to be heated again to 750-950°C for insulation,
At this stage, carbides will be precipitated in the austenite, the stability of the austenite will decrease, and the Ms point will increase above room temperature.
When it is cooled again, a martensite structure is obtained.
Some can also add cold treatment (sub-zero treatment), and then it needs to be aging to make the steel finally obtain strengthened steel with precipitates on the martensite matrix.
For example: In the GB1220 standard, the recommended grade of precipitation 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
- The heating temperature for a solid solution is 1040℃, and the austenite is obtained by water cooling or oil cooling after heating and holding, and the hardness is about 150HB;
- It adjusts the treatment temperature to 760℃, air cooling after heat preservation, which is to precipitate alloy carbides in austenite, reduce the stability of austenite, increase the Ms point to about 50-90℃, and obtain lath martensite after cooling. Hardness can reach about 290HB;
- After aging at 560℃, Al and its compounds precipitate out, the steel is strengthened, and the hardness can reach about 340HB.
Solid solution + adjustment + cold treatment + aging
- Solution heat treatment is at 1040℃ and water cooling is to obtain austenite structure;
- It adjusts the treatment temperature to 955℃, increases the Ms point, and obtain lath martensite after cooling;
- Cold treatment -73℃×8h to reduce the retained austenite in the structure and obtain the maximum martensite;
- The temperature of the aging treatment is 510-560℃ so that Al is precipitated, and the hardness can reach 336HB after strengthening treatment.
Solid solution + cold–deformation + aging
- Solution treatment temperature is 1040℃andwater cooling is to obtain austenite structure;
- Cold deformation uses the principle of cold working deformation strengthening to transform austenite into martensite at the Md point, this cold working deformation is greater than 30-50%;
- Aging treatment: heat aging at about 490°C to make Al precipitate and harden.
- Reports have shown that the hardness of solid solution austenite is 430HB and σb is 1372 N/mm2 after 57% cold rolling.
After aging at 490℃, the hardness is 485HB and σb is 1850 N/mm2.
It can be seen that after the precipitation hardening martensitic stainless steel is properly treated, the mechanical properties can fully reach the performance of martensitic stainless steel, but the corrosion resistance is equivalent to that of austenitic stainless steel.
It should be pointed out here that although both martensitic stainless steel and precipitation hardening stainless steel can be strengthened by heat treatment, the strengthening mechanism is different.
Due to the characteristics of precipitation hardening stainless steel, it has been paid attention and widely used.
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