Controlled Cooling and Thermal Refining Process for Forged and Rolled Steel Utilizing Residual Heat 

Controlled Cooling and Thermal Refining Process for Forged and Rolled Steel Utilizing Residual Heat

Xinfengli Technology Co., Ltd. is a domestic brand specializing in the research, development, and sales of ultra-low temperature cryogenic equipment and various liquid nitrogen cryogenic treatment devices in China. Its primary products include the XFL series liquid nitrogen cryogenic chambers, XFL series liquid nitrogen low-temperature installation equipment, XFL series cryogenic treatment equipment, XFL series high-low temperature test chambers, XFL series mold cryogenic treatment systems, XFL liquid nitrogen ultra-low temperature processing technologies, XFL liquid nitrogen ultra-low temperature treatment skills, and XFL liquid nitrogen ultra-low temperature deep processing machinery. With years of experience in liquid nitrogen low-temperature equipment and liquid nitrogen quick-freezing food machinery, Xinfengli offers comprehensive products and services. Its cryogenic treatment technology has emerged as a novel process for enhancing the performance of metal workpieces in recent years. Cryogenic treatment involves using liquid nitrogen (-196°C) as a refrigerant to further cool quenched metal materials to temperatures far below room temperature. This promotes the further transformationa of residual austenite, present after conventional heat treatment, into martensite, which hardens and strengthens the steel, thereby improving the performance of metal materials. Post-cryogenic treatment, metal tools exhibit significantly enhanced wear resistance, toughness, and dimensional stability, leading to a substantial increase in service life.

Process Overview

Forging heat quenching is a composite thermal processing technique where quenching is performed immediately after high-temperature forging of a workpiece or blank. It essentially involves high-temperature deformation quenching at deformation temperatures (typically 1050-1250°C).

Advantages of the Process

Forging heat quenching streamlines the thermal processing steps in workpiece production, conserves energy, and offers benefits such as improved strength and toughness of steel materials. Compared to conventional heat treatments, forging heat quenching can enhance various mechanical properties of steel: hardness by +10%, tensile strength by +3% to +10%, elongation by +10% to +40%, and impact toughness by +20% to +30%. Furthermore, material exhibits high tempering resistance post-forging heat quenching, with strengthening effects sustainable up to temperatures exceeding 600°C.

Process Parameters

Process parameters significantly influence the strengthening effect, with forging temperature and the holding time between forging and quenching being particularly critical. During high-temperature deformation quenching, the holding time post-deformation and pre-quenching affects steel strengthening, with variations depending on steel composition. Generally, carbon steels or low-carbon alloy steels require immediate quenching post-forging, while medium-alloy steels permit a brief holding time, and high-alloy steels necessitate extended holding periods to achieve optimal mechanical properties. To attain superior strength and toughness, forging temperatures must be precisely controlled to prevent austenite recrystallization during the process. The holding time between forging and quenching is a crucial operational parameter. Experiments indicate that prolonged holding times result in simultaneous reductions in hardness, strength, and impact toughness of the steel, with this decline being more pronounced at higher forging temperatures. Therefore, quenching should be initiated immediately after forging. If operational constraints arise, carbon steels may tolerate a 3-5 second holding time, while alloy steels may require slightly longer durations.

Process Suitability

This process is suitable for die-forged and ZY end-forged workpieces (ZY forging process parameters are independently set, primarily based on final forging temperature and thermal control). For die forging, strict adherence to process flow and operational timings is essential to maintain consistent workpiece residual temperature post-final forging, ensuring uniform quenching performance. This process is applicable to tempered components with general technical requirements, offering superior performance compared to individual tempering.

For certain large workpieces with stringent mechanical property and metallurgical structure requirements, or those requiring extensive machining and subsequent heat treatments, controlled cooling to a specific temperature post-forging, followed by quenching and tempering, may be necessary due to equipment limitations, personnel expertise, material characteristics, or workpiece geometry.

Case Study: 40Cr Forged Connecting Rod for 135 Diesel Engine

The original process involved blanking, die forging (40% deformation), preparatory heat treatment, and tempering. Performance testing revealed average hardness of HRB 250-260, average tensile strength of 799 MPa, elongation of 20.6%, reduction in area of 65.5%, longitudinal impact toughness of 163 J, and transverse impact toughness of 59 J, with surface oxidation and decarburization observed.

Post-modification to residual forging heat quenching, with a forging temperature of 1150°C, the workpiece underwent die forging, trimming, straightening, quenching in an inorganic quenchant, and tempering at 650°C. This resulted in over 75% reduction in surface oxidation and decarburization. Performance testing yielded (average values): hardness of HRB 252-260, tensile strength of 856 MPa, elongation of 19.5%, reduction in area of 66.1%, transverse impact toughness of 94 J, and longitudinal impact toughness of 166 J. Connecting rods produced via residual forging heat underwent a 1800-hour operational test at 179 kW and 2200 rpm, demonstrating stable quality and excellent performance. This modification also reduced energy consumption by 500 RMB per ton of steel by eliminating normalization and reheating for quenching.

Requirements for Online Residual Forging Heat Treatment (LQ Operations)

Upgrading products to premium standards via online residual forging heat treatment imposes specific requirements on forging base materials:

Material Quality: Materials must be free from significant defects, with grain size conforming to national standard grade 4 or finer, and metallurgical structure conforming to national standard grade 3, to prevent cracking due to material defects.
Temperature Control: Residual forging heat should be utilized within the temperature range of 900°C to 770°C, with quenching temperatures determined based on required hardened layer depths and final material performance requirements (which may involve grain refinement for performance enhancement, quenching and self-tempering for performance improvement, or offline martensite tempering for performance enhancement).
Quenching Media: For high-performance medium and high-carbon alloy steel forgings with stringent requirements, inorganic aqueous media are recommended (with quenching capacity of 5 mm/second for material diameters ranging from 16 mm to 60 mm). This ensures deep hardened layers, high hardness uniformity, and effective prevention of thermal shock cracking. Forgings with conventional structural requirements may employ online controlled quenching and self-tempering. For components with rigorous performance requirements, a reheating and tempering process may be adopted. Quenching media temperature should be controlled below 40°C to ensure consistent material performance. Media concentration should be monitored every 15 days, with timely adjustments made. The media requires no water treatment agents, minimal maintenance, and long-term usability. Quenching media costs are controlled below 5 RMB per ton of steel.
Xinfengli Technology Co., Ltd. is a domestic brand specializing in the research, development, and sales of ultra-low temperature cryogenic equipment and various liquid nitrogen cryogenic treatment devices in China. Its primary products include the XFL series liquid nitrogen cryogenic chambers, XFL series liquid nitrogen low-temperature installation equipment, XFL series cryogenic treatment equipment, XFL series high-low temperature test chambers, XFL series mold cryogenic treatment systems, XFL liquid nitrogen ultra-low temperature processing technologies, XFL liquid nitrogen ultra-low temperature treatment skills, and XFL liquid nitrogen ultra-low temperature deep processing machinery. With years of experience in liquid nitrogen low-temperature equipment and liquid nitrogen quick-freezing food machinery, Xinfengli offers comprehensive products and services. Its cryogenic treatment technology has emerged as a novel process for enhancing the performance of metal workpieces in recent years. Cryogenic treatment involves using liquid nitrogen (-196°C) as a refrigerant to further cool quenched metal materials to temperatures far below room temperature. This promotes the further transformation of residual austenite, present after conventional heat treatment, into martensite, which hardens and strengthens the steel, thereby improving the performance of metal materials. Post-cryogenic treatment, metal tools exhibit significantly enhanced wear resistance, toughness, and dimensional stability, leading to a substantial increase in service life.

Technical Specifications:

Operating Temperature Range: +200°C to -196°C (adjustable)
Internal Chamber Temperature: ≤ -196°C
Maximum Cooling Rate: ≥ 50°C/min
Temperature Control Accuracy: ±1°C during heating, cooling, and holding phases
Temperature Uniformity: ±1°C
Cryogenic treatment technology represents a novel process for enhancing metal workpiece performance. By utilizing liquid nitrogen (-196°C) as a refrigerant, it extends the cooling process of quenched metal materials to temperatures significantly below room temperature. This facilitates the further conversion of residual austenite, remaining after conventional heat treatment, into martensite, which hardens and strengthens the steel, thereby improving metal material properties. Post-treatment, metal tools demonstrate marked improvements in wear resistance, toughness, and dimensional stability, leading to a considerable increase in service life.

The benefits of cryogenic treatment permeate the entire workpiece (a holistic effect), ensuring that regrinding for reuse does not negate the treatment's effects. Unlike surface treatments, cryogenic treatment does not alter workpiece geometry or dimensions; instead, it enhances dimensional stability and reduces quenching stresses. The process system is simple, energy-efficient, and applicable to workpieces of various shapes and sizes, with straightforward operation. Furthermore, it is an environmentally friendly and safe technology with no pollution.

Since its emergence, cryogenic technology has garnered significant attention from scientific and industrial communities. In foreign countries, it has been applied to cutting tools, gauges, dies, and precision components such as oil pump nozzles, engine turbine shafts, rolls, valves, gears, and springs. The application of cryogenic modification technology is rapidly gaining traction in the industrial sector. In countries like the United States, Russia, and Japan, cryogenic technology is not only employed to enhance the wear resistance, strength, toughness, and overall service life of high-speed steels, bearing steels, and die steels but also utilized for cryogenic modification of aluminum alloys, copper alloys, cemented carbides, plastics, rubbers, asphalt, and glass, significantly improving their uniformity, dimensional stability, and service life.

For inquiries regarding Xinfengli's cryogenic treatment equipment, please contact Zhang Jianfeng at 13021103555.