Daily Question (20100315): Heat Treatment of CrWMn Steel

Spheroidizing annealing, quenching, cold treatment, and low-temperature tempering. After grinding, artificial aging treatment is also required

The composite heat treatment of CrWMn steel is divided into two steps: pre-treatment and quenching + low-temperature tempering. (a) Conventional annealing, (b) isothermal spheroidizing annealing, (c) cyclic spheroidizing annealing, (d) high-temperature solution treatment + cyclic spheroidizing annealing. After pre-treatment using various processes, specimens with a favorable microstructure and distribution are selected, and they undergo the final heat treatment of quenching + low-temperature tempering at different temperatures. The microstructure and distribution are observed, and changes in hardness are measured. 3 Test Results and Analysis of the Quenching + Tempering Process for CrWMn Steel Microstructure photos of CrWMn steel after various pre-treatment processes; the hardness of CrWMn steel after conventional annealing is 180–190 HB, while it ranges from 180–200 HB after heat treatment. Microstructure of CrWMn steel after pretreatment: (a) conventional annealing, (b) isothermal spheroidizing annealing, (c) cyclic spheroidizing annealing, (d) solution treatment + cyclic spheroidizing annealing. As can be seen from Figure 3, in the microstructure of CrWMn steel after conventional annealing, the carbides are distributed in a flaky pattern ； After isothermal spheroidizing annealing at 810°C, the carbides are distributed in an irregular granular form on the ferritic matrix, with uneven distribution ； After 3 cycles of spheroidizing annealing at 790°C/680°C, the size of the granular carbides decreased and their distribution became more uniform ； After solution treatment at 1050°C followed by spheroidizing annealing in 3 cycles at 790°C/680°C, the carbides precipitate as fine particles with a high degree of dispersion. From a processing perspective, to achieve the same hardness, performing spheroidizing annealing at 790°C/680°C for 3 cycles can not only replace isothermal spheroidizing annealing at 830°C, but also improve the shape and distribution of carbides in the microstructure, reduce the time required for spheroidizing annealing, and save energy. This is because during the heating and holding at temperatures above Ac1 (750°C) in cyclic spheroidizing annealing, the carbides in the flaky pearlite dissolve and break off at their sharp corners; whereas during heating and holding at temperatures below Ar1 (710°C), granular carbides precipitate at the planes of the original flaky carbides. This accelerates the spheroidization process of CrWMn steel and improves the shape and distribution of the carbides. At high temperatures of 1050°C, a large amount of insoluble carbides of alloying elements such as W and Cr in CrWMn steel dissolve into the austenite; after oil quenching, martensite or lower bainite structures are formed. During the subsequent cyclic spheroidizing annealing at 790°C/680°C, punctate carbides of W and Cr precipitate in a dispersed manner. Therefore, for the commonly required CrWMn steel, adopting a spheroidizing annealing process at 790°C/680°C for 3 cycles not only meets the requirements regarding microstructure and hardness but also improves productivity and reduces energy consumption ； For higher requirements, a pre-treatment process of high-temperature solution treatment at 1050°C followed by spheroidizing annealing in 3 cycles at 790°C/680°C can be employed. Microstructure of CrWMn steel after quenching at different temperatures followed by low-temperature tempering: (a) Quenching at 790°C + tempering at 200°C; (b) Quenching at 830°C + tempering at 200°C; (c) Quenching at 870°C + tempering at 200°C; (d) Quenching at 900°C + tempering at 200°C. 4 Conclusions: (1) Using a 790°C/680°C cycle of spheroidization instead of conventional annealing or isothermal spheroidization for CrWMn steel not only improves its microstructural condition and properties, but also enhances the productivity of heat treatment processes and reduces energy consumption. (2) Solid solution treatment at 1050℃ followed by spheroidizing annealing in 3 cycles at 790℃/680℃ can further improve the microstructural distribution of CrWMn steel and enhance its properties. (3) After solution treatment at 1050℃ followed by spheroidizing annealing in 3 cycles at 790℃/680℃, and then oil quenching at 830℃ and tempering at 200℃, the microstructure of CrWMn steel

The heat treatment of gauge steel involves spheroidizing annealing, quenching, cold treatment, and low-temperature tempering. After grinding, artificial aging is also carried out to ensure dimensional accuracy

The heat treatment of gauge steel involves spheroidizing annealing, quenching, cold treatment, and low-temperature tempering. After grinding, artificial aging is also carried out to ensure dimensional accuracy

To ensure dimensional stability during use, it is necessary to eliminate stress in the workpiece as much as possible and reduce the amount of residual austenite. The screw rod is not subject to high forces, but it operates at high speeds; therefore, its surface requires high hardness and wear resistance, with a Rockwell hardness of 60–64 HRC. Given the above requirements for precision lead screws, CrWMn steel is a suitable choice. The reasons are as follows: (1) CrWMn steel is a high-carbon alloy tool steel; after quenching, it achieves high hardness and wear resistance, which can meet the requirements for these properties. (2) Due to the addition of alloying elements, CrWMn steel exhibits good heat treatment properties: it has good hardenability and minimal deformation during heat treatment, which helps to ensure the precision of the screw. Currently, 9Mn2V and CrWMn are used more frequently; however, the former has poorer hardenability and is suitable for precision screws with smaller diameters. Analysis of the original process arrangement: In the original process route, normalizing was not carried out before spheroidizing annealing ； No stress-relief annealing was scheduled after machining ； Reasons such as the absence of cold treatment after quenching and low-temperature tempering cause significant stress and deformation in precision screws during processing, making it difficult to meet their technical requirements. Therefore, the original process sequence should be changed to: material cutting → normalizing → spheroidizing annealing → rough machining → stress-relief annealing → quenching and low-temperature tempering → cryogenic treatment → low-temperature tempering → finish machining → aging → semi-finishing → aging → final finishing.

The heat treatment of gauge steel involves spheroidizing annealing, quenching, cold treatment, and low-temperature tempering. After grinding, artificial aging is also carried out to ensure dimensional accuracy

The heat treatment of gauge steel involves spheroidizing annealing, quenching, cold treatment, and low-temperature tempering. After grinding, artificial aging is also carried out to ensure dimensional accuracy

Steps, Equipment, Temperature, Time and Cooling Method: 1. Stress-relief annealing: Electric furnace, 500°C, 1 hour; cooling in the furnace. 2. Preheating: Electric furnace, 500°C, 5 minutes. 3. Quenching: Salt bath, 820°C; the time required increases with the thickness of the plate. Quenching is carried out in a nitrate bath at 200–220°C; the operator can remove the workpiece for correction before putting it back into the furnace immediately, and then proceed to correct another workpiece. The temperature of the workpieces does not drop, allowing sufficient time for accurate correction after quenching. 4. Make corrections while it’s still hot. 5. Tempering: Use a shaft furnace at 180°C for 3 hours, followed by air cooling. 6. After it reaches room temperature, carry out a second tempering process in the same manner. 1# snifflily

1. Preliminary heat treatment (spheroidizing annealing, stress-relief annealing, quenching and tempering)； 2. Quenching ； 3. Cooling ； 4. Cryogenic treatment ； 5. Tempering ； 6. Aging treatment.

After the rough shape is formed, it needs to be cooled； Cooling is required after finish machining ； After rough grinding, heating is required ； After finishing grinding, it needs to be quenched and tempered.