SE539732C2 - Method of producing a cold drawn wire - Google Patents

Description

METHOD OF PRODUCING A COLD DRAWN WIRE TECHNICAL FIELD The invention relates to a method for the manufacture of a cold drawn wire and wire springs of a precipitation hardenable stainless steel, in particular of the type called 17-7 PH.

BACKGROUND The precipitation stainless steel that contains appr 17 % Cr, appr 7 % Ni, and any precipitation hardening element, normally Al, was developed during the 1940'ies. It was disclosed in an article in the Iron Age, March 1950, pp 79-83. Already in this article, the suitability of the steel as a material for springs was suggested. Good spring features in combination with a good corrosion resistance have made the steel widely used as a spring material in corrosive environments. An environment of that type is injections pumps for Diesel engines, more particularly turbo Diesel engines. Springs which are used for this purpose must have a good corrosion resistance, which 17-7 PH steels have, in combination with a very high fatigue resistance of the springs.

The fatigue resistance depends to a high degree on the surface of the spring wire. In order that the spring shall have a high fatigue resistance, the wire should not have any visible defects, which can initiate fatigue failures. Nor shall the surface layer contain any large slag inclusions or large zones containing major accumulations of smaller slag inclusions, which also can initiate failures.

US 6,383,316 disclose a method for manufacturing a cold drawn wire in which the cast steel is remelted and subjected to an ESR treatment. The ESR ingots are hot worked, which is finished by wire rolling. The rolled wire is pickled and cold drawn. The ESR treatment is employed to avoid large slag inclusions and large zones containing major accumulations of smaller slag inclusions. This was a big improvement compared to prior processes.

DESCRIPTION OF THE INVENTION The present invention proposes a novel route of manufacturing 17-7 PH spring wire and wire springs. The new route includes casting a bulk of molten metal to provide ingots, electro slag refining the ingots for the formation of ESR ingots, remelting the ESR ingots, atomising the remelted ESR ingots to provide a metal powder, hot isostatic pressing the metal powder into a billet, and working the billet into a wire. This new procedure reduces the size of the inclusions further. Furthermore, it essentially removes large zones containing major accumulations of smaller slag inclusions More specifically the, method includes the preparation of a bulk of molten metal, the molten metal comprising in weight %: Image available on "Original document" Optionally max 2 of elements chosen from the group of N, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, TA, B, Be, Bi, Se, Mg, Ca, Hf, V, REM, and, balance Fe apart from impurities.

According to one embodiment of the invention the steel is intentionally alloyed with small amounts of N, preferably 0.005-0.15 % by weight, more preferably 0.01-0.15.

The steel may also be intentionally alloyed with small amounts of Ti, V or Nb. Preferably in weight %: Image available on "Original document" Preferably the total amount of Ti, V or Nb is limited to 0.01-0.2 % by weight.

Preferably the optional elements are limited to (in weight %): Image available on "Original document" Image available on "Original document" REM includes at least one the elements Sc, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Y, Tb, Dy, Ho, Er, Tm, Yb, and Lu.

The bulk of molten metal is cast to the shape of ingots, or, preferably to a strand which is cut up. The ingot or cut-up strand are thereafter electro slag refined, so called ESR remelting, preferably after hot worked to the shape of electrodes.

ESR stands for Electro Slag Refining, also referred to as Electro Slag Remelting. At the ESR treatment there can be used a conventional slag mixture which is used according to known technique, and which at the ESR remelting process forms a melt, in which the electrode that shall be remelted is molten off drop-wise, such that the drops will sink through the slag melt to an underlying pond of molten metal, which solidifies successively to form a new ingot. For example, a slag mixture can be used, which is known per se, and which contains appr 30 % of each of CaF2, CaO, and AI2O3and normally a certain amount of MgO in lime fraction as well as one or a few percent S1O2.

In the case when the melting electrode, as according to the invention, consists of a stainless 17-7 PH steel, which contains slag inclusions of varying sizes, the remelted ingot will get a different slag picture than before the remelting operation. It appears that the ESR slag functions as a screen for larger slag particles existing in the steel prior to the remelting operation. At least this appears to be true for those slags which have proved to have a detrimental effect on the fatigue strength of the spring wire, namely slags of type CaO, AI2O3, and MgO. While the smaller slag inclusions become more evenly distributed and possible zones of slag accumulations become smaller and therefore more harmless, the amount of smaller slag inclusions of this type in the remelted material is influenced only to a low degree.

During the ESR remelting operation, a certain amount of that aluminium, which was added in connection with the initial preparation of the molten metal, can be lost. Therefore, in connection with the ESR remelting operation, more aluminium ought to be supplied to the melting pond for the replacement of any losses, so that the ESR ingot obtained after the ESR remelting operation will contain 0.5-1.5 Al.

The ESR ingots are remelted in a powder production unit, comprising a melting furnace and an atomisation chamber. The remelted ESR ingots are atomised to provide a metal powder. The atomization is preferably by gas atomising. The gas atomisation may be carried out by means of jets of nitrogen and/or of argon gas.

The ESR ingots can be atomised by employing a melting furnace of type where the liquid metal is drained through a drain in the bottom of the furnace to an atomising chamber beneath the furnace. The remelted ESR ingots can be protected in the furnace by an inert gas, vacuum, or slag covering the surface of the remelted ESR ingots. Such furnaces and atomisation chambers are for instance described in US 4,562,943 and WO2013129996, which are hereby incorporated by reference. These furnaces minimise the exposure of oxygen of the molten metal.

An alternative is to have a tiltable furnace and a separate tundish, which both are arranged in an enveloping chamber containing a protective atmosphere. The atomising chamber arranged beneath the tundish. Also in this furnace and tundish combination the oxygen exposure of the molten metal is minimised.

The atomization is preferably by gas atomising. The gas atomisation may be carried out by means of jets of nitrogen and/or of argon gas.

After atomisation, the atomised powder is preferably cooled in a protective atmosphere top avoid re oxidation. Optionally the atomised powder may be sieved to a desired powder gauge. For instance, max 250[ im.

Capsules are filled with the metal powder. After filling, the capsules are sealed. The capsules are thereafter optionally compacted in a cold isostatic press, e.g. Asea QI 100, at a pressure of at least 1000 bar, preferably around 4000 bar. The capsules are thereafter optionally placed in a pre-heating furnace, where the temperature is stepwise risen to a temperature of 900-1250° C, e.g. 1130° C, without being subjected to any externally applied pressure. The capsules are thereafter transferred to a hot isostatic press, e.g. HIPen Asea QI 80, where a pressure at least above 500 bar, e.g. 1000 bar, is applied at a temperature of 900-1250° C, e.g. 1150° C. The compaction of the capsule in the hot isostatic press provides a full density billet. Preferably, the temperature is controlled so that the material is consolidated without presence of liquid phase. The cold isostatic press step as well as the following preheating step are used mainly for process economic reasons and it would very well be possible to transfer the sealed capsules directly to a hot isostatic press without prior cold pressing or preheating.

The billet from the hot isostatic press is thereafter hot worked to rods which are ground and hot rolled to wires. The wires hot rolled to wires are thereafter descaled by mechanical descaling and/or chemical descaling (acid pickling).

The descaled wire is then annealed at a temperature in the range of 900-1100 °C for 0.5 - 2 hours. The annealed wire is cold drawn with at least 30 % area reduction.

The cold drawn wire can be spun to springs, preferably of a helicoidal shape. The springs are suitably precipitation hardened at temperature of 450-500 °C for 0.5- 2 h, followed by cooling in air.

The structure of the material in the finished springs comprises of 50-70 volume-% tempered martensite containing precipitated phases of aluminium and nickel in the martensite, preferably AIM3, remainder austenite and max 5 % 8-ferrite.

The cross sectional shape of the cold drawn spring wires may be circular. The invention, however, is not bound only to wires having such cross section, but can be applied also for wires having other shapes, i.e. wires having oval cross section, which can afford a more favourable distribution of tension in the finished springs which are spun to helicoidal shape. Rectangular cross sections may also be conceivable.

Claims (6)

1. Method of producing a cold drawn wire from a particle metallurgy steel, comprising the following steps:preparation of a bulk of molten metal comprising in weight %: Image available on "Original document" Optionally max 2 of elements chosen from the group of N, P, S, Cu, Co, W, Mo, Nb, Ti, Zr, Ta, B, Be, Bi, Se, Mg, Ca, Hf, V, and REM, and, balance Fe apart from impurities; casting the prepared molten metal to the shape of ingots, or, preferably to a strand which is cut up; electro slag refining, so called ESR remelting, of said ingot or cut-up strand, preferably after hot working to the shape of electrodes, for the formation of ESR ingots; remelting the ESR ingots; atomising the melt of remelted ESR ingots, thereby providing a metal powder; filling a capsule with the metal powder; sealing the capsule; optionally compacting said capsule in a cold isostatic press; optionally preheating said capsule; compacting the capsule in a hot isostatic press to provide a full density billet; hot working the billet and finishing by wire rolling; descaling the resulting rolled wire; annealing the descaled wire; and cold drawing the annealed wire with at least 30 % area reduction.

2. The method according to claim 1, wherein the bulk of molten metal comprises in weight %: Image available on "Original document"

3. The method according to claim 1, wherein the bulk of molten metal comprises in weight %: Image available on "Original document"

4. The method according to any one of claims 1-3, wherein the bulk of molten metal comprises at least one of the following elements: Image available on "Original document" and fulfilling the condition Ti+Nb+V 0.01-0.2.

5. The method according to any one of claims 1-4, wherein the method further comprises: protecting the remelted ESR ingots in an inert gas, vacuum, or by a slag covering the surface of the melt; atomising the remelted ESR ingots by draining the liquid metal through a drain in the bottom of a furnace containing the melt to an atomising chamber.

6. Method for producing springs comprising the steps of: - producing a cold drawn wire according to any one of claims 1-5; spinning springs from the cold drawn wire, preferably to a helicoidal shape; precipitation hardening the springs, preferably at temperature of 450-500 °C for 0.5- 2 h.