Microstructure of high-alloy steel and a heat treatment method of producing the same

Abstract

A method of producing a microstructure of a high-alloy steel includes heating the metal stock to a temperature between 1270° C. and 1280° C., at a rate between 40° C./s and 45° C./s, followed by compression applied to the metal stock in a thixotropic process, after which the stock is cooled to ambient temperature. A microstructure is also shown, which includes undissolved metal carbides in the form of globular particles of austenite microstructure and of martensite microstructure.

Claims

1. Microstructure of a high-alloy steel characterized by its content of undissolved metal carbides in the form of globular particles in the range of 10 wt. %-25 wt. %, 40 wt. %-50 wt. % austenite microstructure and 10 wt. %-25 wt. % martensite microstructure.

2. A method of producing the microstructure of a high-alloy steel in claim 1 characterized by heating the metal stock to a temperature between 1270° C. and 1280° C. at a rate between 40° C./s and 45° C./s, after which compression is applied to the metal stock in a thixotropic process, after which the stock cools to ambient temperature.

3. A method of producing the microstructure of a high-alloy steel in claim 2 characterized by the use of thixoforming after heating to a temperature between 1270° C. and 1280° C.

Description

SUMMARY OF FIGURES

(1) FIG. 1 and FIG. 2 show the resulting microstructure using an optical microscope and FIG. 3 shows the resulting microstructure using a scanning microscope.

EXAMPLE EMBODIMENT

(2) The steel chosen for the experimental example has a chemical composition which is compatible with the proposed processing strategy and enables its implementation. Based on calculations, the CPM 15V steel made by powder metallurgy was chosen. In its basic condition, it consists of vanadium and chromium carbides embedded in a ferritic matrix. This steel possesses high wear resistance and high hardness. Its great weaknesses consist in poor formability and machinability.

(3) TABLE-US-00001 TABLE 1 Chemical composition of CPM 15V steel (wt. %) C Cr V Mo Mn Si 3.40 5.25 14.5 1.30 0.50 0.90

(4) In order to gain more complete understanding of the mechanical properties, a compression test was used, thanks to which the load response of the material can be determined.

(5) In the initial condition, the average measured hardness was 298 HV10. In the thixoformed condition, the hardness was 728 HV10. The same trend was observed in the compression test, where the yield strength upon semi-solid forming increased from the initial value of 627 MPa to 1990 MPa, which represents a threefold increase. This notable increase in compressive yield strength can be attributed mainly to the martensite in the matrix and to the precipitation of chromium in the form of network. The microstructure of the material upon thixoforming consisted of globular vanadium carbides embedded in an austenitic matrix, as shown in FIG. 1 and FIG. 2. X-ray diffraction phase analysis showed that the microstructure of the CPM 15V steel in the centre of the product upon thixoforming at 1270° C. was a mixture of austenite 50%, an iron phase with a body-centred cubic crystal structure 29% and V8C7 vanadium carbides 21%. In the case of the alpha-iron phase, it is martensite. The comparison with the initial condition of the CPM 15V steel revealed that the V8C7 carbides remained present in the microstructure and that the ferritic matrix transformed to austenite and martensite. The occurrence of those carbides in the microstructure provides the products with new potential, e.g. for high wear resistance. Vickers hardness was measured along the entire length of the product.

(6) TABLE-US-00002 TABLE 2 Comparison between compressive yield strength and Vickers hardness parameters Compressive yield strength [MPa] HV10 [—] Initial Thixoformed Thixoformed condition condition Initial condition condition CPM 15V 627 1990 298 728