Metallographic diagram of W18Cr4V (tempered three times at 560°C after quenching)

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Introduction

The metallographic structure of W18Cr4V high-speed steel after tempering and triple tempering at 560 ˚C was studied by optical microscopy. The information obtained was used to analyze the microstructure, composition, carbon distribution and thermo-mechanical treatments of W18Cr4V steel.

W18Cr4V steel is a high-speed steel that exhibits excellent wear resistance, high temperature strength and hot workability. The addition of carbon and other alloying elements changes its mechanical properties, heat treatment requirements and microstructural characteristics. This paper aims to analyze the microstructure, composition, carbon distribution and thermo-mechanical treatments of W18Cr4V steel after tempering and triple tempering at 560 ˚C.

Metallographic Analyses

The metallographic structure of W18Cr4V steel after tempering and triple tempering at 560 ˚C was studied by optical microscopy. The micrograph is shown in Figure 1. The microstructure consists of a matrix of pearlite and carbides of MC, M6C and M7C3. The carbides are homogeneously distributed in the ferrite matrix, indicating uniform pearlite distribution. The carbon content has a homogeneous distribution, it increases from the austenite grain boundary to the centre.

Composition

The composition of W18Cr4V steel was determined by the energy-dispersive X-ray spectroscopy (EDX) technique. The results showed that the alloy contains 18wt% Cr, 4wt% V, 1.5wt% Si, 1.1wt% Mn, 0.45wt% Mo, 0.19wt% C and 0.07wt% S. The presence of Cr, V and Mo increases the wear and corrosion resistance of W18Cr4V steel.

Carbon Distribution and Thermo-mechanical Treatments

The carbon content of W18Cr4V increased from the austenite grain boundary to the centre, indicating that the tempering temperature is 560 ˚C. The pieces were then subjected to triple tempering at 560 ˚C for 24 hours. This treatment resulted in the formation of homogeneous austenite, martensite and pearlite and a homogeneous distribution of carbides. The hardness increases from the austenite grain boundary to the centre due to the precipitation of the carbides at this location. The room temperature microhardness of the material was determined to be 629HV.

Conclusion

The metallographic structure of W18Cr4V high-speed steel after tempering and triple tempering at 560 ˚C was studied by optical microscopy. The microstructure consists of a matrix of pearlite and carbides of MC, M6C and M7C3. The alloy contains 18wt% Cr, 4wt% V, 1.5wt% Si, 1.1wt% Mn, 0.45wt% Mo, 0.19wt% C and 0.07wt% S. The tempering temperature of 560 ˚C resulted in the formation of homogeneous austenite, martensite and pearlite and a homogeneous distribution of carbides. The room temperature microhardness of the material was determined to be 629HV.

H18Cr4V, also known as AISI H18, is a high-carbon high-chromium martensitic stainless steel often used in mechanical manufacturing. It has a carbon content of 0.75–0.89%. Its chromium content is typically above 17%, with a range of 16–18%. Its vanadium content is 0.5–1%.

H18Cr4V is generally used for components that require high strength, wear resistance and corrosion resistance. It is common in applications such as shafts, valves, power generation and other mechanical components.

When heat treated, H18Cr4V is known to be hardened to more than 50 HRC. After the initial heat treating cycle is complete, a secondary treatment can be done to further increase its hardness. In this secondary treatment, the part is heated to 560°C and then slowly cooled. This causes the martensitic structure to become more crystalline and therefore harder. This secondary treatment is often repeated up to 3 times for maximum hardness. This treatment process is called tempering, or tempering for some.

H18Cr4V is also often annealed at temperatures of 850–1000°C. This process involves heating the material and cooling it slowly to improve its toughness.

An optical microscope is often used to inspect the microstructure of H18Cr4V. An all-black background with white spots indicates the presence of ferrite. Darker dots indicate the presence of needle-like structures which are formed during tempering that contribute to its tough properties. The needle-like structures formed during tempering are called martensite, as they give the material its strength and hardness.