CS243252B1 - Method of heat treatment of centrifugally cast castings - Google Patents

Abstract

The invention relates to the heat treatment of castings, in particular metallurgical cylinders and bands, in which at least one working layer is made of chromium cast iron with a chromium content of 12.0 to 27.0 wt. %, carbon of 1.5 to 3.5 wt. %, nickel of 0.1 to 4.5 wt. %, molybdenum of up to 3.0 wt. %, tungsten of up to 3.0 wt. % and vanadium of up to 2.5 wt. %. The casting is gradually heated to a temperature of 400 to 630 °C and remains at this temperature for 1 to 5 hours, based on 20 mm of the thickness of the working layer of the casting, and then cooled.

Description

The invention relates to a method of heat treatment of centrifugally cast castings, in particular cast metallurgical rolls and bandages, in which at least the working layer is of chromium cast iron with 12 to 27 wt. % chromium, 1.5 to 3.5 wt. % carbon, 0.1 to 4.5 wt. % nickel, up to 3.0 wt. % molybdenum, up to 3.0 wt. % tungsten and up to 2.5 wt. % vanadium having austenite in the casting structure.

It is known to heat treat castings of chrome cast iron containing 15 to 72 wt. % of chromium for which high abrasion resistance is required. This property is achieved after heat treatment of centrifugally cast castings, i.e. after heating to a temperature of 1000 to 1200 ° C, after which they are cooled in a cooling medium, for example oil, air and finally tempered at a temperature of 300 to 400 ° C. . This method of heat treatment has a number of drawbacks. It requires a high heating temperature, ie above 1000 degrees Celsius, the heat treatment process takes at least one week for, for example, rollers, and heating temperatures above 1000 ° C are not suitable for many materials, eg gray cast iron with spherical, lamellar or transition graphite, possibly still alloyed and for all types of carbon steels. The latter is very serious, since it is particularly advantageous to combine at least two materials, one of which is always chrome cast iron, in the manufacture of rolls and bandages by centrifugal casting. For example, a combination may be used where the core of the cylinder is cast from gray cast iron or ductile cast iron and the working part of the body is chrome cast iron.

These disadvantages are overcome by the method of heat treatment of the centrifugally cast castings according to the invention. The raw or roughed casting is heated at a temperature of 5 to 30 degrees Celsius per hour to a temperature of 400 to 630 ° C, which remains at this temperature for 1 to 5 hours, based on the 20 mm thickness of the casting layer and then cooled.

The advantage of the solution according to the invention is a low heating temperature. It is not necessary to heat the casting to a temperature of 1000 to 1200 ° C, but only to 630 ° C, which results in energy and capacity savings of the annealing furnaces. Only half the time of the previously used methods is necessary for heat treatment. Furthermore, materials for which high temperatures are undesirable, such as gray cast iron, can be used for the core cores. At the same time, the hardening operation, which creates the risk of cracking of the castings, is removed from the technological process. The plant also has a smaller scale thickness, which has been reduced to 0.5 mm from 3 mm in previously used solutions.

The centrifugally cast casting had, before the heat treatment according to the invention, a structure made of austenite, chromium carbides and martensite. At annealing temperatures up to 500 degrees Celsius austenite decayed to martensite and peak decay reached at about 500 ° C. At higher annealing temperatures, austenite is replaced by martensite and bainite in the material structure. With increasing temperature bainite and martensite decreased. At temperatures above 560 ° C, mainly perlite was present in the sample structure.

Reducing cylinders with a body diameter of 230 mm and a working layer thickness of 16 mm, centrifugally cast into steel shells, were processed by this technology. The cylinders were cast from an alloy with 2.0 wt. % carbon, 0.4 wt. % manganese, 0.4 wt. % silicon,

12.5 wt. % chromium, 1.1 wt. % tungsten, 0.2 wt. % vanadium and 0.2 wt. % nickel. The crude cast was heated at a temperature of 30 ° C per hour to 530 ° C and remained at that temperature for 4 hours. The cylinder was then allowed to cool slowly to a temperature of 60 ° C in the switched-off furnace and was then removed from the furnace. Cooling of the cylinder in the furnace has also been successfully replaced by cooling the cylinder in dry sand. The rollers have a hardness of 480 to 500 HB.

The method of heat treatment of cast iron rolls enables the production of especially double-layer cast rolls intended for profile and sheet rolling mills, both during hot and cold rolling.

Claims (1)

SUBJECT Process for heat treatment of centrifugally cast castings, in particular cast metallurgical rolls and bandages, in which at least a working layer of chrome cast iron with 12.0 to 27.0 wt. % Chromium, 1.5 to 3.5 wt. % carbon, 0.1 to 4.5 wt. % nickel, up to 3.0 wt. % molybdenum, up to 3.0 wt. up to 2.5 wt. % vanadium, characterized in that the raw or roughed casting is heated at a rate of 5 to 30 ° C per hour to a temperature of 400 to 630 ° C and remains at this temperature for 1 to 5 hours, based on 20 mm of the working layer thickness of the casting; cools down.