GB2057510A

GB2057510A - Surface hardening by remelting

Info

Publication number: GB2057510A
Application number: GB7930568A
Authority: GB
Original assignee: AEG Elotherm GmbH
Current assignee: SMS Elotherm GmbH
Priority date: 1979-09-04
Filing date: 1979-09-04
Publication date: 1981-04-01
Also published as: GB2057510B

Abstract

A method of remelt surface hardening of a cast-iron workpiece, preferably an unalloyed cast-iron camshaft, by fusion of the workpiece surface along fusion tracks using an energy beam directed onto the surface followed by cooling of the fused tracks by heat dissipation into lower-lying layers of the material of the workpiece at such a rate that in the resolidified metal of the fused tracks, a ledeburitic structure is formed, characterized in that the said cooling is delayed by supplying heat to the workpiece so that a wholly or partially austenitic layer underneath the solidified ledeburite layer cools at a sufficiently slow rate so that no structural transformation into martensite occurs. After the fusion has been effected, the said cooling is preferably effected by placing the workpiece in a heated furnace. The energy beam is preferably an arc beam. <IMAGE>

Description

SPECIFICATION Method of remelt surface-hardening of castiron workpieces This invention relates to a method of remelt surface hardening of a cast-iron workpiece, by fusion of the workpiece surface along fusion tracks using an energy beam directed onto the surface followed by cooling of the fused tracks by heat dissipation into lower-lying layers of the material ofthe workpiece at such a rate that in the resolidified metal of the fused tracks a ledeburitic structure is formed. The invention is of particular application to camshafts, particularly camshafts made of unalloyed cast-iron.

In known procedures for carrying out such a method, for instance as described in West German patent specification O/S 2344270, the use of an arc in combination with a protective gas is used for effecting track fusion. Furthermore as the hardening of cast-iron workpieces is independent of the method by which they were cast, the workpieces may be hardened using an automatic hardening process whereby the associated structure formation is more easily controllable.

However such known methods suffer from the defect that the workpieces tend easily to form cracks both in their fused and unfused portions.

Such a problem is mitigated by the method of the present invention.

The invention provides a method of remelt surface hardening of a cast-iron workpiece, by fusion of the workpiece surface along fusion tracks using an energy beam directed onto the surface followed by cooling of the fused tracks by heat dissipation into lower-lying layers of the material of the workpiece at such a rate that in the resolidified metal of the fused tracks a ledeburitic structure is formed, characterized in that the said cooling is delayed by supplying heat to the workpiece so that a wholly or partially austenitic layer underneath the solidified ledeburite layer cools at a sufficiently slow rate so that no structural transformation into martensite occurs.

Preferably the workpiece is subjected to a preheating process before the said fusion is effected in such a manner that after fusion has been effected the average workpiece temperature underneath the molten fused tracks after remelting has been carried out remains just below the A, lower transformation point.

After fusion has been effected, the said cooling is preferably effected by placing the workpiece in a heated furnace.

By the procedure of the invention in which the speed of cooling of the workpiece is reduced below the critical speed of a martensitic structure of increased volume underneath the remelted layer which solidifies ledeburitically, is prevented thereby avoiding the generation of stresses in the outer shell of the workpiece which would contribute to crack formation.

Because of the generally high carbon content of cast-iron the critical speed of cooling of such workpieces is very low, and during the cooling step of the present invention the amount of heat supplied, e.g. once or repeatedly during the cooling stage for lowering the speed of cooling, is smaller than the amount of heat which would be released outwards to the environment by the workpiece during natural cooling. The supply of heat to the workpiece during the controlled cooling is preferably effected by placing the workpiece in a heated furnace.

The invention is particularly applicable for castiron workpieces made of unalloyed cast-iron, as alloying constituents e.g. chromium, tend to reduce the critical speed of cooling and thereby increase the risk of crack formation.

One procedure for carrying out the method of the invention is hereinafter described and illustrated in the accompanying drawing. In a first step the track fusion is effected in conventional manner as illustrated schematically in Figure 1, wherein the surface of the workpiece 1 is fused in a track 4 (shown in cross-section) by means of an arc 2 in a screen of protective gas produced via a nozzle 3, the arc 2 being guided over the surface of the workpiece 1. The fusion may be effected for example by means of a device in accordance with West German O/S P 2703469.1 by an arc 2 moved over the surface of the workpiece 1.

Directly after the fusion of the surface of the workpiece 1 in the track 4, which extends over the surface of the workpiece in the form of a fused track, a supplementary amount of heat is fed to the workpiece 1 in an after-heating station, e.g. in a furnace, for reducing the speed of cooling. This step may be repeated. The after-heating may commence during the remelting of the surface of the workpiece.

In Figure 2 the reference 5 schematically designates a pre-heating station in which the workpieces 1 may be pre-heated in accordance with a preferred feature of the invention. The preheated workpieces are subsequently passed to remelting station 6 for the track-fusion of their surfaces, as shown in Figure 1. After such fusion has been effected, the workpieces are passed to a furnace 7 in which they cool at a controlled rate according to the invention.

In such instances in which the workpiece is subjected before the remelting to a pre-heating process for increasing the workpiece temperature, e.g. in a pre-heating station acting conductively.

Such pre-heating is effected up to a maximum temperature which is chosen in'such a way that the average workpiece temperature after remelting has been carried out remains just beneath the transformation point A1, so that a partial formation of martensite is avoided by the nearly adiabatic balance after the remelting step, and by this choice of the pre-heat temperature the elastic limit of the output structure becomes reduced as far as possible so that in the white (ledeburitic) solidified outer layer only correspondingly low shrinkage stresses result.

The supply of heat during the cooling of the remelted workpieces may be performed in a furnace, in afterheating stations or at the location of the track-fusion.

Claims

1. A method of remelt surface hardening of a cast-iron workpiece, by fusion of the workpiece surface along fusion tracks using an energy beam directed onto the surface followed by cooling of the fused tracks by heat dissipation into lowerlying layers of the material of the workpiece at such a rate that in the resolidified metal of the fused tracks a ledeburitic structure is formed, characterized in that the said cooling is delayed by supplying heat to the workpiece so that a wholly or partially austenitic layer underneath the solidified ledeburite layer cools at a sufficiently slow rate so that no structural transformation into martensite occurs.

2. A method according to Claim 1, in which the workpiece is subjected to a preheating process before the said fusion is effected in such a manner that after fusion has been effected the average workpiece temperature underneath the molten fused tracks after remelting has been carried out remains just below the A, lower transformation point.

3. A method according to Claim 1 or Claim 2, wherein the said energy beam is an arc beam.

4. A method according to any of Claims 1 to 3, wherein after the fusion has been effected, the said cooling is effected by placing the workpiece in a heated furnace.

5. A method according to any of Claims 1 to 4, wherein the said workpiece is a cast-iron camshaft.

6. A method according to Claim 5, wherein the said camshaft is made of unalloyed cast-iron.

7. A method according to Claim 1, substantially as hereinbefore described, with particular reference to the drawing.

8. A surface-hardened cast-iron workpiece obtained by a method according to any of Claims 1 to 7.