Heat treatment method for Ni5 martensitic stainless steel forgings

Abstract

The invention discloses a heat treatment method for Ni5 martensitic stainless steel forgings, which comprises the following steps: high-temperature normalizing: preserving heat at 500-600 DEG C, heating to 1020-1060 DEG C at a heating speed of less than or equal to 80 DEG C / h, preserving heat, discharging and air-cooling to room temperature; high-temperature quenching: preserving heat at 500-600 DEG C, heating to 980-1040 DEG C at a heating speed of less than or equal to 80 DEG C / h, preserving heat, discharging from the furnace, and performing oil cooling or cooling in an aqueous solution with the concentration of 10-12% to ensure that austenite is transformed into a martensite structure; high-temperature tempering is carried out twice at the temperature of 550-700 DEG C, the structure stability is ensured, the required good mechanical property is obtained, the overall quality of the interior of a forge piece can be improved and the structure uniformity can be promoted through high-temperature normalizing, and the structure genetic phenomenon of the high-Ni martensitic stainless steel is eliminated by combining high-temperature normalizing and high-temperature quenching and carrying out structure repeated transformation. The method has the advantages that fine grain size is obtained, the problem of uneven grain size is solved, two-time high-temperature tempering is adopted, good structure stability is obtained, internal stress is eliminated, good strength, plasticity and toughness can be obtained, and forging quality is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of heat treatment of metal materials, and particularly relates to a heat treatment method for Ni5 type martensitic stainless steel forgings, which can not only eliminate the inheritance of high-Ni type stainless steel tissue and refine the grain size, but also enable Ni5 type martensitic stainless steel to obtain better comprehensive mechanical properties such as strength, plasticity and toughness, thereby improving the quality of forgings. Background Art

[0002] In actual production, due to the high alloy composition of Ni5 type martensitic stainless steel forgings, not only are surface cracks prone to occur during the forging process, resulting in waste, but also due to the high Ni content, tissue inheritance is prone to occur, resulting in coarse grain size and greater difficulty in production operation. Therefore, in order to ensure the quality of the forging billet, more forging times are performed during the production process, resulting in the forging billet being kept warm at high temperature for a long time, which ultimately causes coarse grains and tissues inside the forging billet. According to the conventional heat treatment production process: quenching + tempering method, it is difficult to effectively eliminate tissue inheritance and improve the coarse grains and tissues inside the forging billet. At the same time, it is impossible to obtain a stable quenched and tempered tissue, and thus it is difficult to obtain good mechanical properties, affecting product quality. In view of the above situation, there is an urgent need for a heat treatment process method to improve the quality of Ni5 type martensitic stainless steel forgings.

[0003] Purpose of the Invention The purpose of the present invention is to overcome the shortcomings of the prior art and provide a Ni5 type martensitic stainless steel forging heat treatment method, which not only eliminates tissue inheritance and obtains fine grains, but also can uniformly organize and obtain good comprehensive mechanical properties, thereby improving the quality of forgings. Summary of the Invention

[0004] In order to achieve the above-mentioned purpose, the present invention adopts the following technical solutions: Step 1) High-temperature normalizing: When loading the furnace, the furnace temperature is ≤450℃, and the temperature is increased to 500℃~600℃ at a heating rate of ≤80℃ / h and kept warm for 1h~10h. Then the temperature is increased to 1020℃~1060℃ at a heating rate of ≤80℃ / h and kept warm for 1h~20h. After that, the forging is taken out of the furnace and air-cooled to room temperature. Then the forging is loaded into the heating furnace for high-temperature quenching. Step 2) High temperature quenching: The furnace temperature is ≤450℃, and the temperature is increased to 500℃~600℃ at a heating rate of ≤80℃ / h for insulation for 1h~10h, and then the temperature is increased to 980℃~1050℃ at a heating rate of ≤80℃ / h for insulation for 1h~20h. After that, the forging is taken out of the furnace for quenching and cooling. First, pre-cooling is performed for 50s~200s, and then oil cooling is performed for 10min~100min (the initial oil temperature during quenching is ≤70℃) or placed in a 10%~12% aqueous solution for cooling for 5min~70min (the initial aqueous solution temperature during quenching is ≤50℃), and then the forging is placed in the heating furnace for high temperature tempering; Step 3) Primary high-temperature tempering: The furnace temperature is set to ≤450°C when loading the furnace, then heated to 550°C~700°C at a heating rate of ≤50°C / h, kept at this temperature for 1h~40h, cooled in oil or water to room temperature, and then the forgings are loaded into the heating furnace for secondary high-temperature tempering; Step 4) Secondary high temperature tempering: When loading the furnace, the furnace temperature is ≤450℃, then the temperature is increased to 550℃~700℃ at a heating rate of ≤50℃ / h, kept at this temperature for 1h~40h, and then air-cooled to room temperature after being taken out of the furnace.

[0005] Compared with the prior art, the process of the present invention has the following advantages: 1. High temperature normalizing can improve the overall quality of the forging and promote the uniformity of the structure.

[0006] 2. Through the combination of high temperature normalizing and high temperature quenching, the organization is repeatedly transformed to eliminate the inheritance phenomenon of high Ni martensitic stainless steel organization, thereby obtaining a fine grain size and effectively solving the problem of uneven grain size.

[0007] 3. The use of two high-temperature tempering processes not only achieves good structural stability and eliminates internal stress, but also achieves better comprehensive mechanical properties such as strength, plasticity, and toughness. The Ni5 type martensitic stainless steel forgings produced according to the present invention not only improve the quality of the forgings. DETAILED DESCRIPTION

[0008] Example 1: A heat treatment method for a Ni5 type martensitic stainless steel forging, chemical composition: C = 0.022%, Si = 0.23%, Mn = 0.47%, Cr = 12.55%, Ni = 5.47%, S = 0.001%, P = 0.019%, Mo = 1.93%, V = 0.07%; specification: Φ330mm*4000mm; heat treatment process is as follows: Step 1) High temperature normalizing: When loading the furnace, heat the furnace to 220°C, increase the temperature to 550°C at a heating rate of 50°C / h and keep it warm for 2 hours, then increase the temperature to 1040°C at a heating rate of 80°C / h and keep it warm for 6 hours, then take it out of the furnace and air cool it to room temperature; Step 2) After step 1) is completed, perform high-temperature quenching: the furnace temperature is 335°C when the furnace is loaded, and the temperature is increased to 550°C at a heating rate of 50°C / h and kept warm for 2 hours, then increased to 1020°C at a heating rate of 80°C / h and kept warm for 6 hours before being taken out of the furnace for quenching and cooling, first pre-cooling for 100 seconds, then oil cooling for 85 minutes, and the initial oil temperature during quenching is 50°C; Step 3) After step 2) is completed, perform a high-temperature tempering: the furnace temperature is 270°C when the furnace is loaded, and then the temperature is increased to 650°C at a heating rate of 50°C / h, kept at this temperature for 12 hours, and then taken out of the furnace and oil-cooled to room temperature; Step 4) After step 3) is completed, perform secondary high-temperature tempering: the furnace temperature is 381°C when the furnace is loaded, and then the temperature is increased to 620°C at a heating rate of 50°C / h, kept at this temperature for 12 hours, and then taken out of the furnace and air-cooled to room temperature.

[0009] After production according to the above heat treatment process, the test results are shown in Table 1: Table 1 Test results Technical requirements Yield strength (Mpa) Tensile strength (Mpa) Elongation (%) Shrinkage rate (%) Impact energy (J) Grain size ≥ level 5 Actual detection 687 866 20 65 189 / 170 6.5 After being produced by the Ni5 type martensitic stainless steel forging heat treatment method of the present invention, the grain size of the forging meets the requirements and obtains good mechanical properties.

[0010] Example 2: A heat treatment method for a Ni5-type martensitic stainless steel forging. Chemical composition: C = 0.023%, Si = 0.25%, Mn = 0.47%, Cr = 12.58%, Ni = 5.50%, S = 0.002%, P = 0.019%, Mo = 1.91%, V = 0.07%. Specifications: Φ330mm * 7800mm.

[0011] The heat treatment process is as follows: Step 1) High temperature normalizing: When loading the furnace, heat the furnace to 360°C, increase the temperature to 550°C at a heating rate of 50°C / h and keep it warm for 2 hours, then increase the temperature to 1040°C at a heating rate of 80°C / h and keep it warm for 6 hours, then take it out of the furnace and air cool it to room temperature; Step 2) After step 1) is completed, perform high-temperature quenching: the furnace temperature is 320°C when the furnace is loaded, and the temperature is increased to 550°C at a heating rate of 50°C / h and kept warm for 2 hours, then the temperature is increased to 1020°C at a heating rate of 80°C / h and kept warm for 6 hours before being taken out of the furnace for quenching and cooling, first pre-cooling for 100 seconds, then oil cooling for 85 minutes, and the initial oil temperature during quenching is 45°C; Step 3) After step 2) is completed, perform a high-temperature tempering: the furnace temperature is 290℃ when loading, and then the temperature is increased to 650℃ at a heating rate of 50℃ / h, kept at this temperature for 12h, and then taken out of the furnace and oil-cooled to room temperature.

[0012] Step 4) After step 3) is completed, perform secondary high-temperature tempering: the furnace temperature is 277°C when the furnace is loaded, and then the temperature is increased to 620°C at a heating rate of 50°C / h, kept at this temperature for 12 hours, and then taken out of the furnace and air-cooled to room temperature.

[0013] After production according to the above heat treatment process, the test results are shown in Table 2: Table 2 Test results Technical requirements Yield strength (Mpa) Tensile strength (Mpa) Elongation (%) Shrinkage rate (%) Impact energy (J) Grain size ≥ level 5 Actual detection 667 847 22 65 181 / 193 6.5 After being produced by the Ni5 type martensitic stainless steel forging heat treatment method of the present invention, the grain size of the forging meets the requirements and obtains good mechanical properties.

Claims

1. A heat treatment method for Ni5 type martensitic stainless steel forgings, characterized by: The specific steps are as follows: Step 1) High temperature normalizing: When loading the furnace, the furnace temperature is ≤450℃, and the temperature is increased to 500℃~600℃ at a heating rate of ≤80℃ / h and kept warm for 1h~10h. Then the temperature is increased to 1020℃~1060℃ at a heating rate of ≤80℃ / h and kept warm for 1h~20h. After that, the forging is taken out of the furnace and air-cooled to room temperature. Then the forging is loaded into the heating furnace for high temperature quenching. Step 2), high temperature quenching: when loading the furnace, the furnace temperature is ≤450℃, and the temperature is increased to 500℃~600℃ at a heating rate of ≤80℃ / h for insulation, and the temperature is kept for 1h~10h, and then the temperature is increased to 980℃~1050℃ at a heating rate of ≤80℃ / h for insulation, and the temperature is kept for 1h~20h. After that, the forging is taken out of the furnace for quenching and cooling, first pre-cooling for 50s~200s, and then oil cooling for 10min~200min. When quenching, the initial oil temperature is ≤70℃ or placed in a 10%~12% aqueous solution for cooling for 5min~100min. When quenching, the initial liquid temperature of the aqueous solution is ≤50℃, and then the forging is loaded into the heating furnace to prepare for a high temperature tempering; Step 3) Primary high-temperature tempering: The furnace temperature is ≤450℃ when loading, then the temperature is raised to 550℃~700℃ at a heating rate of ≤50℃ / h, kept at this temperature for 1h~40h, cooled by oil or water to room temperature after being taken out of the furnace, and then the forgings are loaded into the heating furnace for secondary high-temperature tempering; Step 4) Secondary high temperature tempering: When loading the furnace, the furnace temperature is ≤450℃, then the temperature is increased to 550℃~700℃ at a heating rate of ≤50℃ / h, kept at this temperature for 1h~40h, and then air-cooled to room temperature after being taken out of the furnace.