Heat treatment method for ultrahigh-strength stainless steel

Abstract

The invention relates to a heat treatment method for ultrahigh-strength stainless steel, and belongs to the technical field of metal materials. The ultra-high-strength stainless steel comprises, by mass, 0.15-0.25% of C, 11.0-13.50% of Cr, 2.00-3.80% of Ni, 11.00-14.00% of Co, 3.50-5.50% of Mo and 0.80-1.50% of W. The method comprises the steps that A, an ultra-high-strength stainless steel workpiece is heated to 1060-1100 DEG C, heat preservation is conducted for 60 min + / -5 min, oil cooling is conducted, quenching oil for oil cooling requires that the hardness of a standard 45 # steel quenching sample with the aging resistance within 3 years is reduced by not more than 20% compared with that of the standard 45 # steel quenching sample, and the quenching cooling characteristic is that the maximum cooling rate is larger than or equal to 90 DEG C / s; and the maximum cooling speed interval is 600-700 DEG C. The product quality is remarkably improved.

Description

Technical Field

[0001] The present invention relates to a heat treatment method for ultra-high strength stainless steel, belonging to the technical field of metal materials. Background Art

[0002] A certain type of ultra-high strength stainless steel with chemical composition containing wt%: C 0.15 - 0.25, Cr 11.0 - 13.50, Ni 2.00 - 3.80, Co 11.00 - 14.00, Mo 3.50 - 5.50, W 0.80 - 1.50 is widely used in marine environment due to its high strength, good fracture toughness and excellent corrosion resistance, and is a new material for manufacturing load-bearing and corrosion-resistant structural components. This material obtains a strength level of 1900 MPa through low-carbon martensite transformation strengthening and dispersion strengthening of metal carbide M2C, and the toughness of the steel is greatly optimized by aging treatment to obtain reverse transformation austenite. At the same time, adding 13% of Cr improves the corrosion resistance of the steel.

[0003] The heat treatment system of a certain type of ultra-high strength stainless steel is: "1070°C - 1090°C, holding for 60 min ± 5 min, oil quenching; -65°C - -81°C, holding for 120 min ± 5 min, warming in air. 540°C - 550°C, holding for 4.5 h, air cooling". In engineering applications, it is required that the forgings reach the following indicators after sample heat treatment: sampling direction longitudinal, σ b / MPa ≥ 1900, σ 0.2 / MPa ≥ 1530, K IC / MPa·m 1 / 2 ≥ 80. However, in the actual production process, there are situations of low tensile strength margin and low qualified rate, which seriously affect the product quality. Summary of the Invention

[0004] The purpose of the present invention is to provide a new heat treatment method for ultra-high strength stainless steel.

[0005] To achieve the purpose of the present invention, the composition of the ultra-high strength stainless steel is by mass percentage: C 0.15 - 0.25, Cr 11.0 - 13.50, Ni 2.00 - 3.80, Co 11.00 - 14.00, Mo 3.50 - 5.50, W 0.80 - 1.50, and the method includes:

[0006] A. Heating the ultra-high strength stainless steel workpiece to 1060°C - 1100°C, holding for 60 min ± 5 min, and oil quenching to room temperature. The quenching oil for the oil quenching is required to have an anti-aging ability such that the hardness ratio reduction of the standard 45# steel quenching sample within 3 years is not more than 20%, and the quenching cooling characteristics: maximum cooling rate ≥ 90°C / s, maximum cooling speed range: 600 - 700°C;

[0007] B. Cool the workpiece after oil cooling in step A at -82°C to -92°C for 360 min ± 5 min for cold treatment, and then warm it back to room temperature in air.

[0008] C. Age the workpiece after warming in step B at 540 - 550°C for 7 - 10 h, and then perform air cooling.

[0009] In a specific embodiment, the ultra-high strength stainless steel workpiece is a mechanical property specimen or an actual part.

[0010] In a specific embodiment, the effective cross-section of the ultra-high strength stainless steel workpiece is ≤ 30 mm.

[0011] In a specific embodiment, the spacing between the ultra-high strength stainless steel workpieces in step A is 5 - 30 mm.

[0012] In a specific embodiment, the heating temperature accuracy of the ultra-high strength stainless steel workpiece in step A is controlled within ±5°C.

[0013] Preferably, the quenching oil for oil cooling in step A is B-244.

[0014] In a specific embodiment, the cold treatment in step B is carried out at -87°C for heat preservation, and the heat preservation accuracy is ±5°C.

[0015] In a specific embodiment, the medium for cold treatment in step B uses liquid heat conduction.

[0016] In a specific embodiment, the medium for cold treatment is alcohol.

[0017] In a specific embodiment, the heat preservation time in step B is 360 min.

[0018] Beneficial effects:

[0019] The present invention increases the degree of cold treatment, and at the same time, the strengthening phases are in a coherent state under specific aging temperatures, and the strength increases within the aging time of the present invention.

[0020] The present invention, without modifying the existing equipment, according to the embodiment

[0021] Multiple production tests prove that the improved process increases the product qualification rate from 70% to 90%, significantly improving the product quality.

[0022] The present invention reduces the number of repeated heat treatments, reduces the manufacturing cost, and does not use high-energy-consuming refrigerants such as liquid nitrogen throughout the process. Description of the drawings

[0023] Figure 1Flow chart of heat treatment for embodiments of the present invention;

[0024] Figure 2 Tensile specimen diagram;

[0025] Figure 3 For K IC Specimen diagram. Specific embodiments

[0026] To achieve the object of the present invention, the composition of the ultra-high strength stainless steel is by mass percentage: C 0.15 - 0.25, Cr 11.0 - 13.50, Ni 2.00 - 3.80, Co 11.00 - 14.00, Mo 3.50 - 5.50, W 0.80 - 1.50, and the method includes:

[0027] A. Heat the ultra-high strength stainless steel workpiece to 1060°C - 1100°C, hold for 60 min ± 5 min, and oil cool to room temperature. The quenching oil for the oil cooling is required to have an anti-aging ability such that the hardness ratio reduction of the standard 45# steel quenching specimen within 3 years is not more than 20%. Quenching cooling characteristics: maximum cooling rate ≥ 90°C / s, maximum cooling speed range: 600 - 700°C;

[0028] B. Cold treat the workpiece after oil cooling in step A at -82°C - -92°C, hold for 360 min ± 5 min, and then warm back to room temperature in air;

[0029] C. Age the workpiece after warming back in step B at 540 - 550°C, hold for 7 - 10 h, and then air cool.

[0030] Heating the workpiece to 1060°C - 1100°C, holding for 60 min ± 5 min, and oil cooling has two significances. Firstly, it prepares for the transformation of the matrix austenite into martensite. Secondly, there are a large number of M6C phases and χ phases in the steel before heat treatment, and it provides a basis for their solution to form secondary phase precipitation. Since the dissolution temperature of M6C phase and χ phase is higher than the austenitizing temperature, the heating temperature of this process is higher than the general quenching temperature, and it is more appropriate to call it solution.

[0031] As is well known, martensite transformation is affected by the thickness of the specimen and the quenching medium. The thicker the specimen thickness, the worse the quenching effect. Practical engineering research has found that for this material, when the equivalent thickness of the specimen increases by 10 mm, the strength decreases by 1%. In addition, in existing production, for the same batch of specimens, the performance fluctuates greatly during continuous processing with the same process and the same equipment. The present invention has carried out in-depth research on this material, and the specimens processed have the best performance, good anti-aging characteristics, and small performance differences for continuously processed specimens. It solves the problem that in existing production, for the same batch of specimens, the performance fluctuates greatly during continuous processing with the same process and the same equipment, and the qualification rate of specimen processing reaches more than 90%.

[0032] In a specific embodiment, the ultra-high strength stainless steel workpiece is a mechanical property specimen or an actual part.

[0033] In a specific embodiment, the effective cross-section of the ultra-high strength stainless steel workpiece is ≤ 30 mm.

[0034] In a specific embodiment, the spacing between the ultra-high strength stainless steel workpieces in step A is 5 - 30 mm. In industrial production, products are generally processed in groups of several or dozens. Different placement methods have a direct impact on the heat penetration time of the workpieces and the cooling effect during quenching. Considering the comprehensive aspects of quenching uniformity and industrial production cost, a spacing of 5 - 30 mm between workpieces is preferable.

[0035] In a specific embodiment, the heating temperature accuracy of the ultra-high strength stainless steel workpiece in step A is controlled within ±5°C. There is a certain deviation between the actual heating temperature in different regions and the set temperature. Too low a temperature will cause incomplete dissolution of M6C phase and x phase, and too high a temperature will cause coarse grains. Controlling the furnace temperature accuracy within ±5°C can improve the effect of furnace temperature uniformity. A furnace equipped with an in-furnace fan can be used for heating to achieve forced convection and control the furnace temperature accuracy within ±5°C.

[0036] Preferably, the quenching oil for oil cooling in step A is B-244 quenching oil.

[0037] In a specific embodiment, the cryogenic treatment in step B is to hold at -87°C with a holding accuracy of ±5°C.

[0038] Tests have shown that for the specimens with the improved cryogenic treatment, the content of retained austenite is 1.5%, while for those with the original process, the content of retained austenite is 3%. The original reverse Carnot cycle commercial refrigeration equipment can be used. Refrigerants prepared with high energy consumption such as liquid nitrogen are not used.

[0039] In a specific embodiment, the medium for the cryogenic treatment in step B uses liquid heat conduction.

[0040] In a specific embodiment, the medium for the cryogenic treatment is alcohol.

[0041] In a specific embodiment, the holding time in step B is 360 min.

[0042] The following further describes the specific embodiments of the present invention in conjunction with the examples, and the present invention is not limited to the scope of the described examples.

[0043] The experiment of the present invention uses a certain type of precipitation hardening high-strength stainless steel, and the composition is wt%: C 0.10, Cr 11.93, Ni 2.97, Co 12.93, Mo 4.52, W 1.01. The forgings made of this steel are required to have high strength (σb≥1900 MPa), high toughness (K IC ≥80 MPa·m 1 / 2 ), and the advantages of being stainless.

[0044] Example 1

[0045] Four tensile test specimens and two fracture toughness specimens made of the ultra-high strength stainless steel involved in the present invention are used. The effective cross-section of the tensile specimens is 10 mm, and the fracture toughness specimens are 30 mm. They are heated to 1075 °C at a spacing of 5 - 30 mm, and the furnace temperature accuracy is controlled within ±5 °C. After holding for 60 min, they are quenched and cooled with 32# quenching oil until room temperature. Then the treated workpieces are put into a cold treatment box with a specimen spacing of 5 - 30 mm. The cold treatment set temperature is -87 °C, and it is held for 360 min. After that, they are taken out of the cold treatment box and gradually warmed to room temperature in the air, and then aging treatment is carried out. The aging temperature is 555 °C, the holding time is 7 h, and air cooling is performed.

[0046] Example 2

[0047] Four tensile test specimens and two fracture toughness specimens made of the ultra-high strength stainless steel involved in the present invention are used. The effective cross-section of the tensile specimens is 10 mm, and the fracture toughness specimens are 30 mm. They are placed at a spacing of 5 - 30 mm, then heated to 1085 °C, the furnace temperature accuracy is controlled within ±5 °C, held for 60 min, then cooled to room temperature with B-244 quenching oil, and then cold treatment is carried out. The workpiece spacing is 5 - 30 mm, the cold treatment set temperature is -87 °C, held for 360 min, and then taken out of the furnace and returned to room temperature in the air, and then aging treatment is carried out. The workpiece spacing is 5 - 30 mm, the aging temperature is 555 °C, the holding time is 7 h, and air cooling is performed.

[0048] Example 3

[0049] Four tensile test specimens and two fracture toughness specimens made of the ultra-high strength stainless steel involved in the present invention are used. The effective cross-section of the tensile specimens is 10 mm, and the fracture toughness specimens are 30 mm. They are placed at a spacing of 5 - 30 mm, then heated to 1080 °C, the furnace temperature accuracy is controlled within ±5 °C, held for 60 min, then cooled in B-244 quenching oil, and then cold treatment is carried out. The cold treatment temperature is -87 °C, and after holding for 360 min, it is taken out of the furnace and returned to room temperature in the air. Aging treatment is carried out, the aging temperature is 555 °C, the holding time is 10 h, and air cooling is performed.

[0050] Comparative Example 1

[0051] Four tensile test specimens and two fracture toughness specimens made of the ultra-high strength stainless steel involved in the present invention, with the effective cross-section of the tensile specimens being 10 mm and that of the fracture toughness specimens being 30 mm, are placed at intervals of 5 - 30 mm, then heated to 1080 °C, the furnace temperature accuracy is controlled within ±5 °C, held for 60 min, then cooled in 40# quenching oil, and then cold-treated at a cold treatment temperature of -87 °C. After holding for 360 min, it is taken out of the furnace and restored to room temperature in the air. Aging treatment is carried out at an aging temperature of 555 °C for 7 h, and air-cooled.

[0052] Comparative Example 2

[0053] Four tensile test specimens and two fracture toughness specimens made of the ultra-high strength stainless steel involved in the present invention, with the effective cross-section of the tensile specimens being 10 mm and that of the fracture toughness specimens being 30 mm, are placed at intervals of 5 - 30 mm, then heated to 1080 °C, the furnace temperature accuracy is controlled within ±10 °C, held for 60 min, and then cooled in 32# rapid quenching oil; then cold-treated at a cold treatment temperature of -73 °C, and after holding for 120 min, it is warmed to room temperature in the air; then aged at an aging temperature of 555 °C for 270 min.

[0054] Examples 1 - 3 and Comparative Examples 1 - 2 are compared as shown in Table 1:

[0055] Table 1 Main performance index table of precipitation hardening ultra-high strength steel after heat treatment

[0056]

[0057]

[0058] Judging from the data in Table 1, the process disclosed in the present invention can obtain forgings with various performance meeting the requirements, and the strength and toughness indexes of the forgings are significantly improved; the strength and toughness data of the workpieces are more stable than the existing process. Forty groups of tensile specimens and fracture toughness specimens made of ultra-high strength stainless steel are processed according to Comparative Example 1 and Comparative Example 2 respectively. After testing the tensile and fracture toughness data, the qualified rate of Comparative Example 1 is 65% and that of Comparative Example 2 is 70% according to the technical standards. Forty groups of tensile specimens and fracture toughness specimens made of ultra-high strength stainless steel are processed in total according to Example 1, Example 2, and Example 3. After testing the tensile and fracture toughness data, the qualified rate is 90% according to the technical standards. It can be seen that the product performance is significantly improved.

Claims

1. A heat treatment method for ultra-high strength stainless steel, wherein the composition of the ultra-high strength stainless steel is, by mass percentage, C 0.15-0.25, Cr 11.0-13.50, Ni 2.00-3.80, Co 11.00-14.00, Mo 3.50-5.50, W 0.80-1.50, characterized in that: The method comprises: A. Heat the ultra-high strength stainless steel workpiece to 1060℃~1100℃, keep it warm for 60min±5min, and oil-cool it to room temperature. The oil-cooled quenching oil is required to have an anti-aging ability of not more than 20% reduction in hardness of the standard 45# steel quenching sample within 3 years. The quenching cooling characteristics are: maximum cooling rate ≥90℃ / s, and maximum cooling speed range: 600~700℃; B. After oil cooling in step A, keep the workpiece at -82℃~-92℃ for 360min±5min for cold treatment, and then return the temperature to room temperature in air; C. After the workpiece is warmed up in step B, keep it at 540-550℃ for 7-10 hours for aging treatment, and then air cool it.

2. The heat treatment method for ultra-high strength stainless steel according to claim 1, characterized in that: The ultra-high strength stainless steel workpiece is a mechanical property test specimen or an actual part.

3. The heat treatment method for ultra-high strength stainless steel according to claim 2, characterized in that: The effective cross-section of the ultra-high strength stainless steel workpiece is ≤30 mm.

4. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The spacing between the ultra-high strength stainless steel workpieces in step A is 5 to 30 mm.

5. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The heating temperature accuracy of the ultra-high strength stainless steel workpiece in step A is controlled within ±5°C.

6. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The quenching oil for oil cooling in step A is B-244 quenching oil.

7. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The cold treatment in step B is to keep the temperature at -87°C with a temperature accuracy of ±5°C.

8. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The medium for the cold treatment in step B is liquid heat conduction.

9. The heat treatment method for ultra-high strength stainless steel according to claim 8, characterized in that: The medium for the cold treatment is alcohol.

10. The heat treatment method for ultra-high strength stainless steel according to claim 1 or 2, characterized in that: The insulation time in step B is 360 minutes.

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