Effect of deep cryogenic treatment on the mechanical properties of tool steels

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Abstract

The effect of deep cryogenic treatment (−196°C) on the properties of some tool steels was studied by means of both field tests on real tools and laboratory tests. The execution of the deep cryogenic treatment on quenched and tempered high speed steel tools increases hardness, reduces tool consumption and down time for the equipment set up, thus leading to cost reductions of about 50%. A laboratory investigation on an AISI M2 and an AISI H13 steel confirms the possibility of increasing the wear resistance and toughness by carrying out the treatment after the usual heat treatment.

Introduction

Cold treating is widely used for high precision parts and components, since it enhances the transformation of austenite to martensite. The common practice identifies −60 to −80°C as the optimum temperature, according to material and the quenching parameters. Many companies use this kind of treatment to improve surface hardness and thermal stability.

Deep cryogenic treatment in the range −125 to −196°C improves certain properties beyond the improvement obtained by normal cold treatment [1], [2], [3], [4]. The main reason for this is the complete transformation from austenite into martensite plus the formation of very small carbides dispersed in the tempered martensitic structure [5]. The greatest improvement in properties is obtained by carrying out the deep cryogenic treatment between quenching and tempering. However, a significant improvement can be obtained even by treating the tools at the end of the usual heat treatment cycle, i.e. the finished tools. This last solution is more flexible than the other one and can extend the use of the treatment to many practical applications.

In order to check the potential of the deep cryogenic treatment on the performances of the finished products, field tests were carried out on some high speed steel tools, and the results were recorded for a long time, in order to get reliable data from the industrial production. At the same time, a specific laboratory investigation was carried out on two steels: the AISI M2 high speed steel and the AISI H13 hot work tool steel. These experiments were aimed at the study of the effect of the treatment on some mechanical properties and on the wear resistance of the two steels. In the laboratory tests, the deep cryogenic treatment was carried out at different stages of the usual heat treatment route, i.e. both after quenching and after tempering. The results of the tests are presented and discussed here.

Section snippets

Field tests on HSS

Fig. 1 is a schematic representation of the cryogenic equipment. It comprises an insulated box (cryo box), one motor with a circulating fan, one thermocouple to measure the cryogenic temperature inside the box connected to a temperature controller and programmer, a liquid nitrogen tank and a solenoid valve for the gas inlet. The actual temperature of the mass loaded in the box is recorded by a thermocouple inserted in a 30 kg steel block.

Fig. 2 shows the cryogenic cycle: the thin and the bold

Laboratory tests on HSS

Tests were carried out on an AISI M2 steel. The base treatment was the usual one, comprising vacuum quenching from 1220°C and two tempering cycles at 550°C for 2 h each (A). The deep cryogenic treatment was carried out both after the two temperings (B) and after quenching; in this case one (C) and two (D) tempering cycles were carried out. Table 2 summarises the specimens investigated (Q is the quenching, T the tempering, and C the deep cryogenic).

Wear tests were carried out on an Amsler

Laboratory tests on hot work tool steels

Tests were carried out on an AISI H13 hot work tool steel. The base treatment comprises vacuum quenching from 1020°C and two tempering cycles at 570°C for 3 h each (A). Also in this case, the deep cryogenic treatment was carried out both after the two temperings (B) and after quenching; again one (C) and two (D) tempering cycles were carried out on the quenched and deep cryogenic treated steel. Therefore, Table 3 summarises the AISI H13 specimens investigated, as well.

First of all, hardness,

Conclusions

The deep cryogenic treatment (−196°C) of quenched and tempered high speed steel tools improves their properties; in particular, it increases the hardness and improves the hardness homogeneity, reduces the tool consumption and the down time for the equipments set up, thus leading to about 50% cost reduction. In addition to the field tests, laboratory tests were carried out on two different steels in order to study the effect of the deep cryogenic treatment on some mechanical properties and on

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