EP0271688A1

EP0271688A1 - Process for obtaining bainitic nodular cast iron castings

Info

Publication number: EP0271688A1
Application number: EP87116105A
Authority: EP
Inventors: Giuseppe Sola
Original assignee: Necchi SpA
Current assignee: Necchi SpA
Priority date: 1986-11-20
Filing date: 1987-11-02
Publication date: 1988-06-22
Also published as: IT8642915A0; IT1216347B

Abstract

Process for obtaining bainitic nodular cast iron castings based on the severe narrowness of the alloy analytic values and on a particular thermic cycle based in turn on austenitizing at high temperature, after having effected a superificial nitriding treatment, and on isothermic transformation at low temperature with intermediate temper in water at a temperature lower than the transformation temperature.

Description

Disclosure of the invention

The present invention refers to a process for obtaining bainitic nodular cast iron castings.
The bainitic structure is obtained as a rule from a standard cast iron with analytic field decidedly wider especially with reference to the manganese, the molybdenum, the copper and the nickel.
Afterwards a heat treatment in two phases is executed: the first an austenitizing phase with temperature lower than 860 °C; the second a isothermic transformation phase between 350 and 400°C. The structures obtained by a productive cycle of this type do not permit at all the achieave tensile strengths, hardnesses and fatigue behaviours required for amplifying the application field of the so obtained castings.
It is the object of the present invention to overcome the above described drawbacks.
The technical problem to be solved was:
- the obtaining of superior resistances and hardnesses in presence of toughness values proportionally superior to the values obtained with the traditional systems;
- the obtaining of structures with "pitting" superficial resistances comparable to those of the best cement steels which have always had a better behaviour with respect to the bainitic austempered cast irons tested till now;
- the obtaining of fracture and torsion resistances superior to those obtained with the normal already known methods.
The solution of the technical problem is characterized by the fact that the process comprises the following sequences:
- heating of the castings till 500-600 °C and execution of a superficial nitriding treatment;
- further heating till 870-950 °C for the austenitizing treatment of the matrix;
- quick cooling of the castings in a water bath at a temperature between 70 and 300 °C;
- bainitic transformation in a bath of salts between 225 and 350 °C;
- shot peening of the castings at °Almen controlled; said process being applied when the analitic values of the alloy elements correspond to Mn≦0,20%, Ni≦1,0%, Cu≦1,0% and Mo≦0,5%.
Further characteristics and advantages will become more readily apparent from the following description and the enclosed drawing in which the diagram of the thermic cycle object of the present invention is illustrated.
The particularities of this process consist in a strict definition of the analytic field of the alloy elements with Mn≦0,20%, Ni≦1,0%, Cu≦1,0%, Mo≦0,5% and in the application of a particular austenitizing-temper-isothermic transformation cycle combined with a superficial nitriding of the piece. The nodular cast iron castings are abtained with a perlitic- ferric matrix by whatever production technique usually employed. After mechanical working, the castings are subjected to austenitizing treatment of the matrix which occurs by heating the castings at a temperature comprised between 870 and 950 °C for a period comprised between 1 and 3 hours. When the temperature of 500-600 °C is reached, the heating is interrupted for a period variable from 0 to 24 hours in order to effect a treatment of superficial nitriding.
In figure it has been indicated with A the zone of the diagram of the thermic cycle where the treatment of superficial nitriding occurs.
After said nitriding treatment, the temperature is raised for completing the austenitizing cycle. In figure it has been indicated with B the zone of the diagram where the austenitizing treatment occurs.
After said austenitizing treatment, a cooling is effected as fast as possible by immersing the castings in a water bath at a temperture comprised between 70 and 300 °C for a period proportional to the temperature itself and comprised between 1 and 5 minutes, effecting in this way a quick temper. In figure it has been indicated with C the zone of the diagram where the temper in water occurs at a very low temperature. After this quick temper, the bainitic transformation of the austenite is effected in salts bath at a temperature between 225 and 350 °C for a period comprised between 0,5 and 8 hours. In figure it has been indicated with D the zone in which this transformation occurs.
Subsequently the castings are subjected to shot peening and °Almen controlled.
The process object of the present invention is based on the severe narrowness of the alloy analytic values, in particular with reference to manganese, nickel, copper and molybdenum, and on a particular thermic cycle based in turn on austenitizing at high temperature and on isothermic transformation at low temperature with intermediate temper in water at a temperature lower than the transformation temperature.
The results are decidedly superior to those relative to the steel castings, even if induction tempered steel, and to the forged pieces, even if carburized.
Furtherly, with the nitriding effected on the cast iron castings within the austenitizing cycle, it is obtained a high resistance to the abrasion and to the rolling fatigue. This process permits to obtain bainitic structures of cast iron in presence of low percentages of alloy and thus easily obtainable.

Claims

1. Process for obtaining bainitic nodular cast iron castings where said cast iron castings are composed by carbon and by alloy elements, characterized by the fact that it comprises the following sequences:
- heating of the castings till 500-600 °C and execution of a superficial nitriding treatment;
- further heating till 870-950 °C for the austenitizing treatment of the matrix;
- quick cooling of the castings in a water bath at a temperature between 70 and 300 °C;
- bainitic transformation in a bath of salts between 225 and 350 °C;
- shot peening of the castings at °Almen controlled; said process being applied when the analytic values of the alloy elements correspond to Mn≦ 0,20%, Ni≦ 1,0%, Cu≦ 1,0% and Mn≦ 0,5%.

2. Process according to claim 1, characterized by the fact that the superficial nitriding treatment is effected in a period which varies from 0 to 24 hours.

3. Process according to the claim 1, characterized by the fact that the austenitizing treatment occurs in a period which varies from 1 to 3 hours.

4. Process according to the claim 1, characterized by the fact that the quick temper treatment occurs in a period comprised between 1 and 5 minutes and is proportional to the temperature of the water bath.

5. Process according to the claim 1, characterized by the fact that the bainitic transformation of the austenite in bath of salts occurs in a period which varies from 0,5 to 8 hours.

6. Process for obtaining bainitic nodular cast iron castings as hereinbefore described and illustrated in the enclosed drawing.