Deep cryogenic treatment applied in high-speed steel yielded promising results, and it was found that tempering played a vital role. This paper systematically studied the effect of tempering temperatures on the microstructural composition (residual austenite, martensite, and carbides) and mechanical properties (Vickers hardness and impact toughness) of AISI M35 high-speed steel subjected to deep cryogenic treatment. An increase in the tempering temperatures facilitated the transformation of residual austenite and the formation of martensite blocks corresponding to a decrease in the number of dislocations. Moreover, the carbide precipitation (secondary carbides and nanoscale carbides) began at a tempering temperature of 350 °C, increased at 450 °C, and reached its maximum at 550 °C. The fracture mechanism on the micro-level could be interpreted as follows: cracks occurred in the carbides with larger sizes (primary carbides and large secondary carbides) and at the carbide/matrix interface, and small secondary carbides decohered at the interface, forming microvoids and facilitating plastic deformation. In addition, the specimen tempered at 150 °C exhibited the highest hardness of 880.4 HV 1 due to the highest number of dislocations. The impact toughness of the sample tempered at 550 °C was the best, namely 2.50 MJ m −2 , due to an increase in the number of martensite block boundaries and the more homogeneous carbide precipitation. 

Microstructural variations and mechanical properties of deep cryogenic treated AISI M35 high-speed steel tempered at various temperatures

As a different step from conventional heat treatment, cryogenic treatment played a very important role in improving the microstructure and properties of metals. The effect mechanism of cryogenic treatment on steel and non-ferrous metals and their weldments was reviewed. The influence parameters of cryogenic treatment were discussed, and the influence of cryogenic treatment on microstructure was analyzed, which led to the change of tensile strength, corrosion resistance, fatigue resistance, wear resistance and other properties. In addition, the relationship between cryogenic treatment and heat treatment were also discussed. On this basis, the influence mechanism of cryogenic treatment on metal materials was explained from the three aspects of crystal defect, phase transformation and precipitated phase, and the reasons for these phenomena were more deeply understood after cryogenic treatment. On the basis of cryogenic treatment, the method combining with conventional heat treatment and surface treatment was put forward, which provided a new way for obtaining high performance materials. It could be widely used in metal forming and other fields, and provide a broader vision for material post-processing through the prediction of numerical simulation. 

Effect mechanism of cryogenic treatment on ferroalloy and nonferrous alloy and their weldments: a review

High-speed steel is widely used for cutting tools due to its convenience of preparation and cost-effectiveness. Previous research has shown that deep cryogenic treatments improve the mechanical properties of high-speed steel, due to the transformation of the residual austenite and the precipitation of carbide, while few studies have researched martensitic changes. The variations in martensite multi-level microstructures in AISI M35 high-speed steel, treated over different deep cryogenic time periods, were investigated in this study. Meanwhile, the effect of these variations on the mechanical properties of the selected steel was discussed. It was found that prolonging deep cryogenic time facilitated an increase in dislocation, low-angle grain boundary, and the coincident-site lattice boundary (especially the twin boundary) of martensite. The size of the martensite block (db) and lath (dl) decreased with deep cryogenic time. However, the effect on the microstructure was limited when the cryogenic treatment time exceeded 5 h. The increase in dislocation decreased the temperature for carbide precipitation and promoted fine carbide precipitation during tempering. The refinement of martensite multi-level microstructures and the greater precipitation of fine carbides gave the tempered specimens excellent impact toughness. The impact toughness of the tempered samples undergoing deep cryogenic treatment for more than 5 h was about 32% higher than the sample without deep cryogenic treatment. 

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Effect of Deep Cryogenic Time on Martensite Multi-Level Microstructures and Mechanical Properties in AISI M35 High-Speed Steel

Hot-compression bonding with subsequent soaking treatment is employed to manufacture large high-speed homogeneous steels in the current study. The effects of the deformation temperature (1050, 1100, and 1150 °C), strain (10%, 30%, and 50%), and holding time (1 and 12 h) on the interfacial microstructure and tensile strength of joints were investigated. The results indicated that complete bonding occurred at 1050 °C/50%, 1100 °C/30%, 1100 °C/50%, 1150 °C/30%, and 1150 °C/50%, and the tensile strength of the obtained joints was comparable to that of the base material. In other cases, interfacial voids were formed along the straight interface, which deteriorated the bonding strength. These voids were eliminated by a post-holding treatment at 1150 °C for 12 h, promoting the transformation of a straight interface to bulged grain boundaries. This resulted in the complete healing of the interface, except for samples bonded at 1050 °C/10%. The bulging of the original bonding interface due to the plastic deformation and atomic diffusion was the main mechanism for interfacial healing. Stable interfacial oxide nanoparticles were formed under a high vacuum and did not affect the recovery of joint strength. To validate the practicality of this method, an integrated high-speed steel with a homogeneous tensile strength was manufactured by bonding four pieces of blanks. 

A hot-compression bonding method for manufacturing large high-speed homogeneous steels

The microstructure and wear behavior of S390 high-speed steel (HSS) reinforced with different volume fractions of MC-type carbides produced via spark plasma sintering were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM) in this study. SEM and TEM results show that V-W-rich carbides are formed around the added MC-type carbides, and these carbides have a similar composition to the M(C, N) carbides precipitated at high temperatures according to thermodynamic calculations. Both macrohardness and three-point bending results show that the carbide type is the dominant factor increasing the hardness, and the volume fraction of the carbide is the dominant factor leading to a decrease in the three-point bending strength. The wear mechanism of HSS metal matrix composites (MMCs) is confirmed as abrasive wear and oxidative wear via wear tracks and oxidation films. Compared with the sample without reinforcement (85 HRA, wear coefficient of 1.50 × 10−15 m2/N), the best MT-3 sample exhibits a hardness increase of 1.8 HRA and a three-fold increase in wear resistance.

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The Effect of MC-Type Carbides on the Microstructure and Wear Behavior of S390 High-Speed Steel Produced via Spark Plasma Sintering

Effect of quenching, tempering and deep cryogenic treatment (DCT) on microstructure and impact toughness of a new tungsten-molybdenum high-speed steel produced by powder metallurgy was investigated. The obtained results infer that the microstructure is mainly composed of martensite, M6C and VC. The impact toughness decreases with the increasing of austenitizing temperature, but when the austenitizing temperature increases to 1180 °C, the impact toughness changes little. DCT after quenching improves impact toughness slightly, but DCT followed tempering has little effect on impact toughness. The fracture micromechanism can be identified as that crack initiates at the interface between M6C and martensite caused by the inconsistency deformation, M6C carbides exhibit broken or cleavage fracture and VC carbides decohere at the interface. In addition, the variation of impact toughness value has been explained by the characteristics of martensite, the fracture ductility, compressive strain of martensite lattice and grain refinement are beneficial to impact toughness.

Yao Yao

Papers

Effect of heat treatment on microstructure and impact toughness of a Tungsten-Molybdenum powder metallurgical high-speed steel

Effect of fabrication process on wear and impact toughness of high speed steel