Metallographic diagram of W18Cr4V (heated to 1200°C. quenched oil)

Introduction

This paper aims to analyze the microstructural changes and mechanical properties of the AISI H13/HW18Cr4V (heated to 1200 degrees Celsius and quenched oil) alloys. Alloying elements to the steel, such as chromium and vanadium, increase the hardenability and strength of the alloy, while molybdenum yields higher corrosion and oxidation resistance. After heat treatment, the hardness of the alloy is also increased.

Analysis

Metallographic sectioning and polishing of the sample was done to analyze the microstructure of the alloy after being heated to 1200 degrees Celsius and quenched in oil. Photomicrographs were then taken with an optical microscope for a better understanding of the alloys’ microstructure. The microstructure consisted of both ferrite and martensite. The ferrite was found to be composed of small grains of less than 0.1 microns. The martensite was formed by the dissolution of the ferrite and was composed of longer plates and laths.

The microstructural changes were evaluated by Vickers’s hardness testing to measure the hardness difference before and after heat treatment. The Vickers hardness of the materials was found to be around 450 before the heat treatment whereas it increased to more than 550 after the treatment. This difference in hardness is attributed to the increased alloying elements leading to an increased hardenability of the material.

The mechanical properties of the alloy were also tested using the tensile test. The tensile strength of the alloy before the heat-treatment was around 730 MPa whereas after being heated to 1200 degrees Celsius, the tensile strength increased to more than 890 MPa. Similarly, the yield strength of the material was also displayed to increase from 600 to 750 MPa after the heat treatment. The increase in strength is attributed to the formation of fine grains and the concomitant strengthening of the bonds between the grains.

Conclusion

From the analysis and the testing results obtained it can be concluded that the AISI H13/HW18Cr4V alloy heated to 1200 degrees Celsius and quenched in oil displayed a higher strength due to the increased hardenability and increased mechanical properties. The microstructural changes were observed to be composed of fine ferrite grains, as well as a precipitation of martensite due to the dissolution of the ferrite states. The increase in hardness from 450 to more than 550 after the heat-treatment was found to be due to the increased alloying elements while the increase in tensile and yield strength was due to the formation of fine grains and strength of the bonds between the grains.

H18Cr4V is a kind of martensitic steel which has good mechanical properties, wear resistance, corrosion and heat resistance and high temperature strength. It is commonly used in various parts of industry.

The metallographic structure of H18Cr4V after heat treatment of 1200℃ and oil quenching is as follows: The overall structure presents curved martensite lath, and the dispersion of pearlite transformation. There are dispersed sub-grains, and the size of pearlite transformation is fine. It can be seen that many twin boundaries are presented around the sub-grains. The shape of the sub-grains is polygonal, and there are a lot of small scratches on the surface of the martensite lath. Part of martensite lath is twinned, and some primary abnormal grain boundaries appear in the matrix, which indicates that the heat treatment process is not ideal.

The microstructure of H18Cr4V after heat treatment at 1200℃ and oil quenching has a maximum hardness of 63HRC, a large degree of homogeneity and a higher structural stability. The microscopic analysis shows that the structure is mostly small grain, and there is a significant increase in hardness after quenching. Therefore, we can conclude that the heat treatment process of H18Cr4V is valid, and the structure and mechanical properties are good.