DE3018454A1 - Heat treatment of steel parts, esp races for rolling bearings - using brief immersion in liq. nitrogen to reduce residual austenite in surface - Google Patents

Abstract

The part is heated to form austenite, and is then quenched, followed by tempering is required to reduce brittleness. The surface region only of the part is next cooled in a liq. gas, esp. in N2, which is pref. applied as a sprayed mist. The invention is designed to eliminate residual tensile stresses in the surface zones of the parts. By selective sub-zero cooling of the surface zones, some of the residual austenite in these zones is converted to martensite, which creates an increase in vol., producing desirable compressive stresses working in opposition to the stress applied when the part is in use.

Description

Process for the heat treatment of workpieces made of steel,

in particular roller bearing rings The invention relates to a method for the heat treatment of workpieces made of steel, in particular rolling bearing rings which the workpiece is heated to a sufficient austenitizing temperature, quenched, then optionally reheated to reduce the brittleness and through a low-boiling, liquefied gas, preferably nitrogen, is cooled.

When it comes to heat treatment of workpieces made of steel, not only should cooled, but also hardened.

There are therefore - in addition to the cooling heat stresses - additional stresses from the transformation of the structure.

During the cooling process, residual tensile stresses build up. Upon reaching of the martensite point suddenly there is an increase in volume due to the transformation, the compressive stresses. These compressive stresses must be balanced in the core area , which thereby comes under tensile stress. Another voltage reversal takes place, even if the core area has cooled below the martensite point and is below Volume enlargement converts.

But since the outer shell is already hard, it can do this stretching of the core area no longer degrade by flowing and therefore comes under tensile stress. The internal stress curve typical for mertensitic hardening therefore shows tensile stresses in the edge area of the workpiece, which transform into internal compressive stresses in the core.

When the workpiece, in particular a roller bearing ring, is loaded there is then an unfavorable superposition of the residual tensile stresses with the stress resulting from the stress on the workpiece and thus to a total higher load, which can even exceed the permissible load.

Process for the heat treatment of workpieces made of steel, in which the retained austenite is converted into martensite by low temperature treatment, are already known. However, all of the retained austenite is always used converted into martensite, with tensions in the workpiece being avoided as far as possible up to now should be.

The invention is based on the object of specifying a method with the help of which it is possible to produce workpieces made of steel, in particular roller bearing rings to be treated in such a way that, given sufficient hardness, there are no residual tensile stresses in the edge area more are available.

The invention solves the task set in that the cooling only in the edge area of the workpiece.

-Depending on the necessary conversion temperature, the Workpiece can be cooled down for different lengths of time. The lowest possible cooling temperature corresponds to the boiling point of the liquefied gas; - for- nitrogen 77.4 K at 1013 mbar. Conventional 1z1agerstah1 heat-treated in the usual way has about 8 - 10% retained austenite, most of it when it cools down to about 77 K. is converted into martensite.

This conversion, which is limited to the edge area only, results an increase in volume, so that the previously existing tensile internal stresses are reduced in the edge area and even the desired compressive stresses occur.

It is about creating compressive stresses in the edge area as much as possible. For this it is necessary to convert the austenite into martensite in the edge area, whereas the original structure is to be retained in the core area.

In a further embodiment of the invention, the edge area is cooled with nitrogen spray. With such a spray there is a possibility to vary the mixing ratio of liquid and gaseous components and thereby also to change the heat transfer coefficient from the workpiece to the cooling medium or a to achieve exact adaptation to the edge area in which the compressive stresses occur should.

The invention is based on the following exemplary embodiment explained in more detail: A roller bearing made of material no. 1. 2067 (100 Cr 6) with a diameter of 90 mm and 200 mm in length was heated to 8700 C for 30 min kept this Austnitization temperature, then quenched in 600 g warm oil and finally heated to 1800 C tempering temperature. At this temperature it was held the roll for an hour. A sample was then taken and a residual austenite content found by 12%.

The roller was then immersed in liquid nitrogen while rotating, the immersion time being 10 seconds. A subsequent renewed investigation showed that the residual austenite content within an edge area of 3 mm did not exceed 6%, whereas in the core area the retained austenite content remained unchanged at 12%.

Claims (2)

P a t e n t a n s p r ü c h e 1. Process for the heat treatment of Workpieces made of steel, in particular roller bearing rings, in which the workpiece is on sufficient austenitizing temperature heated, quenched, then if necessary to reduce the brittleness, reheated and replaced by a low-boiling, liquefied one Gas, preferably nitrogen, is cooled, characterized in that the cooling only goes on in the edge area of the workpiece. 2. The method according to claim 1, characterized in that the cooling the edge area is carried out with nitrogen spray.