THIS INVENTION relates to welding. In particular, it relates to a welding consumable, to a metal object which includes a weld deposit layer, and to a method of overlay welding a metal object.

Surfacing techniques have been used in a variety of applications for many years but it is only since the 1940's that electric arc welding has been used. The manufacturing industry worldwide is facing the problem of reducing wear of equipment at optimal costs. Surfacing techniques such as weld build-up, hard facing, weld cladding and buttering can be used to extend the life of metal components subjected to thermal shocks, corrosion, friction, fatigue and/or erosion. One particular area in which surfacing techniques have found application has been the reconditioning of steel mill rolls to extend the service life of the rolls to three to five times the normal life of uncladded rolls. For example, the processing rolls of hot strip steel mills are generally subjected to attack by high temperatures, abrasion and corrosion which occur simultaneously. These rolls come into contact with red hot material, abrasive mould scale, and contaminated acidic water and steam. Roll surface deterioration typically occurs due to thermal fatigue cracking, corrosion and abrasion. Any one of these deterioration mechanisms could dominate the type of surface damage, depending on the properties of the surface alloy and the severity of the attack. Roll failure is based on the extent of surface damage and usually comprises an unacceptable wear pattern (which imprints permanent marks on rolled products) or excessive wear (which affects the mechanical operation of the roll) for each type of roll.

Numerous standard materials have been used in the past to manufacture and recondition the equipment used in steel mills, by using welding consumables to

deposit layers on the equipment. However, none of the materials satisfies all of the following criteria to a desirable degree: the weld deposit layer must resist the aforementioned operating conditions adequately ; the welding consumable used to apply the weld deposit layer must be manufacturable at a low cost; the weld deposit layer must have good weldability ; and deposition and machining of the weld deposit layer must be achievable at a low cost.

It is an object of this invention to provide a welding consumable suitable for the overlay welding of a metal substrate, and a welding method, and a cladded metal object, which provide an improvement in respect of one or more of the abovementioned criteria compared to conventional welding consumables, methods or cladded articles.

According to one aspect of the invention, there is provided a welding consumable suitable for the overlay welding of a metal substrate, the welding consumable comprising C, Mn, Cr, Mo and Fe and being characterised in that, when the welding consumable is deposited in a layer on a copper substrate under an inert atmosphere, the weld deposit layer comprises the following constituents, by weight percent, 0,17%-0,27% C, 1,8%-2,8% Mn, 9,0%-12,0% Cr and 0,8%-1,6% Mo with the balance being mainly Fe, and disregarding any copper in the weld deposit layer.

According to another aspect of the invention, there is provided a metal object which includes a weld deposit layer which comprises the following constituents, by weight percent, 0,17%-0,27% C, 1,8%-2,8% Mn, 9,0%-12,0% Cr and 0,8% - 1, 6% Mo with the balance being mainly Fe.

According to a further aspect of the invention, there is provided a method of overlay welding a metal object, the method including depositing a weld deposit layer

on the metal object, the weld deposit layer comprising the following constituents, by weight percent, 0,17%-0,27% C, 1,8%-2,8% Mn, 9,0%-12,0% Cr and 0,8%- 1,6% Mo with the balance being mainly Fe.

In this specification, the term"overlay welding"is used broadly to include weld surfacing processes such as weld build-up, hard facing, weld cladding, buttering, hard surfacing, and the like. Furthermore, in this specification, unless otherwise indicated, all compositions are provided on a percentage by weight basis.

Preferably, the weld deposit layer comprises 0,2%-0,26% C, e. g.

0,23% C, 2,0%-2,6% Mn, e. g. 2,3% Mn, 9,5%-11,5% Cr, e. g. 10,5% Cr, and 0,8%-1,2% Mo, e. g. 1,0% Mo.

The weld deposit layer may include Si. Preferably, the weld deposit layer comprises less than 1 % Si, e. g. 0,2%-0,8% Si.

The weld deposit layer may include V. Preferably, the weld deposit layer comprises maximum 0,3% V.

The weld deposit layer may include Nb. Preferably, the weld deposit layer comprises maximum 0,3% Nb.

The weld deposit layer may include N, which may be present in the layer in a concentration of between 0,01% and 0,14%.

The weld deposit layer may have a hardness of between 48 and 56 when measured according to the Rockwell Hardness Scale C.

The weld deposit layer may be characterised in that it has a martensitic transformation which starts below 200°C. The martensitic transformation may be complete above 25°C, e. g. between about 60°C and about 25°C.

The weld deposit layer may be characterised in that it can have a good tempering resistance at temperatures up to 600°C and/or that it has good non-scaling properties in air at temperatures up to 600°C.

The welding consumable may be a wire. Preferably, the welding consumable is a cored wire. The cored wire may include a mild steel sheath, typically comprising 0,01 %-0,3% C, less than 1 % Si, less than 3,01% Mn, less than 1,5% Mo and less than 0,5% Ni, with the balance being mainly Fe.

The core of the wire may include a powdered composition which, when molten with the sheath provides the weld deposit as hereinbefore described. The powdered composition may include a flux, which is typically a basic flux.

Basicity or acidity of a flux are determined by the various oxides present in the flux. Flux that contains a large portion of silica is generally acid. Flux that contains little silica is generally basic. A"Basicity Index" (B. I.) was defined by the International Institute of Welding (IIW) as: B i _ CaO + MgO + BaO + SrO + Na20 + K20 + Li, O + CaF2 + 0.5 MnO + 0.5 FeO<BR> B. I. _ SiO, + 0.5 Al203 + 0.5 TiO2 + 0.5 ZrO2 where the weight percent of each constituent is entered in the formula above.

A flux whose B. I. is less than 1.0 is termed"acid". A flux whose B. I. is greater than 1.5 is termed"basic". A flux whose B. I. is between 1.0 and 1.5 is termed"neutral", although this latter term can lead to some confusion because"neutral"is also used as a term for a flux which does not produce much change in deposit Mn and Si when large voltage changes occur during welding.

The wire may have a diameter of between 1,6mm and 5mm and may be in the form of a roll.

The metal object may be a cylindrical steel component, e. g. a roller of a steel mill. Thus, the metal object may be selected from the group consisting of slabbing rolls, scale breakers, backup rolls, pinch rolls, leveller rolls, table rolls, cold mill roll journals, vertical edger rolls, run-out table rolls, roughing rolls, breakdown rolls, bar mill rolls, hot strip mill rolls, finishing rolls, caster rolls, and coiling rolls.

The metal object may include a plurality of weld deposit layers. Typically, the last or surfacing layer is the weld deposit layer having the constituents as hereinbefore described. One or more of the weld deposit layers may be a butter pass, and one or more of the weld deposit layers may be a built-up layer.

Depositing the weld deposit layer may be effected with an electric arc welding process. The electric arc process may be selected from the group consisting of metal active gas (MAG), metal inert gas (MIG), flux cored arc, submerged arc, shielded metal arc, and gas metal arc. Typically, the electric arc welding process is selected from the group consisting of metal active gas, flux cored arc and submerged arc.

Preferably, the polarity of electric arc current when using the submerged arc welding process is DC negative, i. e. a welding consumable used to form the weld deposit is negative and the metal object is positive.

The method may include preheating the metal object. Typically, the metal object is preheated to a temperature of at least 200°C, e. g. between 200°C and 250°.

The method may include maintaining an interpass temperature for the metal object during welding of between 200°C and 400°C.

The method may include subjecting the metal object to heat treatment, e. g. tempering and/or stress relieving heat treatment.

When the weld deposit layer is applied in the presence of a flux, the flux may be a basic flux.

The invention will now be described, by way of example, with reference to the following Examples.

Example 1: A cored wire welding consumable in accordance with the invention was manufactured. The wire included a sheath of mild steel with a powdered alloy core.

A mild steel plate was cladded using the welding consumable and a submerged arc welding process. A commercially available basic flux was used during the cladding of the mild steel plate. The welding process was DC negative, i. e. the welding consumable formed the negative electrode. Three layers of cladding were deposited on the mild steel plate.

A sample was taken from the third weld deposit layer. The following table provides the weight percent composition of the sample : Table : C 0.254% Si 0.50% Mn 2.46% P 0.019% S 0.017% Cu 0.07% Al 0.020% Cr 9.89% Mo 0.97% Ni 0.06% V 0.04% Ti <. 01 %

Nb <. 01% Co 0.06% W 0.05% Fe Balance Example 2: The welding wire of Example 1 was used to hard surface a coil wrapper roll of the Iscor Steel Plant in Vanderbijlpark, South Africa. The hardfaced coil wrapper roll was returned to duty. On inspection of the coil wrapper roll one year later, it was found that the roll showed wear of about 0.075mm per month on the diameter of the roll, compared to coil wrapper rolls hard surfaced with conventional welding consumables which showed wear of 1,0mm to 1,2mm per month on diameter.

Example 3: Three scale breaker pinch rolls at the Iscor Plant in Vanderbijlpark, South Africa were hard surfaced with the welding wire of Example 1 and a submerged arc process. After eighteen months, the rolls were still in use. This is in sharp contrast to rolls hard surfaced with conventional welding consumables, which were usually scrapped after six to nine months of use.

Example 4: The coil box bending rolls of the Saldanha Steel Plant, Saldanha, South Africa were hard surfaced with the welding wire of Example 1 and a submerged arc process. The rolls showed no noticeable wear after six months of use, in sharp contrast to the 3mm wear on diameter shown by rolls hard surfaced with conventional welding consumables.

Example 5: The pinch rolls and deflector rolls of the Steckel mill of the Columbus Stainless Steel Plant, Middelburg, South Africa were hard surfaced with the welding wire of Example 1 and a submerged arc process. The hard surfaced rolls remained in duty for more than twelve months. Previously, these rolls, when hard surfaced with

conventional welding consumables, exhibited 3mm to 6mm wear on diameter per year or serious surface damage.

Similarly, at the same plant, a black plate leveller was hard surfaced with the welding wire of Example 1. The black plate leveller did not show any noticeable signs of wear or surface damage after one year of use.

Example 6: In an example of the application of the welding consumable of the invention on objects used in cooler environments, scrap baller rolls at the Columbus Stainless Steel Plant, Middelburg, South Africa were hard surfaced with the welding wire of Example 1 and a submerged arc process. After a year the rolls showed wear of about 1 mm to 2mm on outside diameter. Again, this is in sharp contrast with scrap baller rolls hard surfaced with conventional welding consumables, which typically lasted three months and showed 50mm to 60mm wear on outside diameter over the three month period.

It is an advantage of the invention, as exemplified, that it provides a welding consumable which can be used to hard surface or clad metal objects, providing a surface layer which can resist conditions conducive to thermal fatigue cracking, corrosion and abrasion, particularly the processing rolls of hot strip steel mills. Furthermore, the surface layer provided by the welding consumable has good weldability. The surface layer can be applied cost effectively and machining of the deposited surface layer is readily carried out cost effectively. A further particularly important advantage of the welding consumable of the invention, as exemplified, is that, due to its composition, it is substantially cheaper to manufacture than conventional welding consumables providing weld deposit layers having similar characteristics of durability and machinability to the welding consumable of the invention.

The welding consumable of the invention thus provides a weld deposit layer with:

a hardness between 48 and 56 Rockwell Hardness Scale C, providing good wear resistance; a fully martensitic micro structure for maximum strength and hardness; good resistance to high temperature softening and scaling in air up to 600°C ; excellent thermal fatigue cracking resistance; good general corrosion resistance; excellent solidification cracking resistance during welding, generally obviating the need for buffer layers to prevent cracking; low preheat and interpass temperature requirements during welding (only above 200°C) to ensure full martensitic transformation after welding and cooling ; the characteristic that it does not generally need post weld heat treatment; a martensitic transformation which starts below 200°C and which is complete just above room temperature; quick and easy hot rough machinability of the weld deposit layer when kept above 200°C after welding, as a result of the soft austenitic structure at this temperature, even if kept above 200°C for one week.