This invention relates to a powder composition for surface-coating metal substrates by plasma transfer, i.e. by the so-called PTA process (Plasma Transferred Arc), as well as a method for surface-coating a metal substrate with the powder composition. In particular, the invention is directed to a powder composition and a method suitable for surface-coating the rollers in the hot zone of a machine for continuous casting.

To increase the hardness, the strength and the oxi¬ dation resistance of metal substrates, in particular low- alloy steel substrates, it is known to surface-coat them with alloys having superior such properties than the sub- strates themselves. Usable alloys include those based on cobalt and available under the trade name "Stellite".

This invention aims at providing a surface coating which above all exhibits good high-temperature properties upon substantial temperature changes, as is required in the above-mentioned application. In this context, oxida¬ tion resistance, high-temperature strength and thermal- shock resistance are of particular importance.

According to the invention, this aim is generally achieved by an iron-base powder composition which, in addition to Fe, contains C, Ni, Cr and, optionally, one or more of Mn, Si, Mo, N and 0. To be more specific, the inventive composition should contain 18.0-30.0% by weight of Cr, 5.0-20.0% by weight of Ni and 1.0-3.5% by weight of C, the balance being Fe. When Mn, Si and Mo are included in the composition, they may each make up 0.1-3.0% by weight. N may be included in an amount of at least 0.005% by weight, typically 0.05% by weight, but larger amounts are also conceivable. Since the composition is powder- based, 0 may be included in an amount of up to 0.05% by weight, but as little as 0.005% by weight can be used.

The lower limit for the Cr content is preferably 20.0% by weight, most preferred 22.0% by weight. The upper limit for the Cr content is preferably 28.0% by weight, most preferred 26.0% by weight. The lower limit for the Ni content is 7.0% by weight, most preferred 9.0% by weight. The upper limit for the Ni content is preferably 17.0% by weight, most preferred 14.0% by weight. The lower limit for the C content is preferably 1.2% by weight, but may be 1.5% by weight. The upper limit for the C content is pre- ferably 3.2% by weight, most preferred 2.8% by weight.

According to the invention, it is essential that the surface coating has an austenite phase in the entire tem¬ perature range concerned up to at least 1000°C, which is reliably achieved by the indicated amounts of C and Ni. The absence of transitions between cc-phase and 2f-phase along with a high content of chromic carbide would seem responsible for the excellent high-temperature properties obtained.

Also, the inventive powder composition provides high mechanical integrity with excellent resistance to high- temperature corrosion.

In the inventive method, the powder, whose size is in the range of 45-250 um, usually 63-210 μm, and alter¬ natively 53-150 μm or 45-125 μm, is deposited, preferably by the PTA process, in which a transferred arc, in contra¬ distinction to plasma spraying, reaches the substrate and partly melts its surface layer.

A surface coating produced with the aid of the inven¬ tion is advantageous also in that it does not necessitate any after-treatment, apart from optional stress relieving. The invention will be illustrated in more detail below by a comparison between the properties of three sur¬ face coatings prepared from three different powder compo¬ sitions A, B and C, reference being made to the accompany- ing drawings, in which

Fig. 1 illustrates the hardness,

Fig. 2 illustrates the coefficient of thermal expan¬ sion, and

Fig. 3 illustrates the increase in weight. Composition A

This powder composition was used as reference compo¬ sition and consisted of 0.09% by weight of C, 1.1% by weight of Ni, 14.0% by weight of Cr and 0.7% by weight of Mn, the balance being Fe. Composition B

This composition was based on cobalt ("Stellite F" ) and consisted of 1.7% by weight of C, 22.7% by weight of Ni, 0.6% by weight of Fe, 25.6% by weight of Cr, 12.3% by weight of W, 0.5% by weight of Mn and 1.3% by weight of Si, the balance being Co. Composition C

This inventive powder composition consisted of 2.4% by weight of C, 11.3% by weight of Ni, 24.0% by weight of Cr, 0.6% by weight of Mo and 1.2% by weight Si, the balance being Fe.

Fig. 1 illustrates the hardness (HV 20) of the three surface coatings as a function of temperature. Thus, the surface coating based on inventive composition C had a hardness comparable to that of the surface coating based on composition B and is clearly superior to that of the surface coating based on composition A.

The diagram in Fig. 2 illustrates the coefficient of thermal expansion of the three surface coatings as a func¬ tion of temperature. The surface coatings based on compo- sition B and C show a substantially continuous change of the coefficient of thermal expansion, whereas the surface coating based on composition A shows a decrease in the coefficient of thermal expansion at 800°C owing to a change of phase. The inventive coating being austenitic within the entire temperature range, such an undesirable change of the coefficient of thermal expansion is avoided. The different developments of the coefficient of thermal

expansion shown in Fig. 2 manifest themselves in diffe¬ rent thermal-shock resistance. By changing the tempera¬ ture of the coatings between 100°C and 800°C at a rate of four times per minute, cracks formed much faster and to a larger extent in the surface coating of composition A than in the surface coatings of compositions B and C.

The diagram in Fig. 3 illustrates the increase in weight of the three surface coatings as a function of time at a temperature of 1200°C. The increase in weight is an indication of the oxidation resistance, i.e. the smaller the increase in weight, the better the oxidation resis¬ tance. As appears from Fig. 3, the surface coating of reference composition A is much poorer than the surface coatings of compositions B and C. As is evident from the foregoing, the properties of the inventive composition are, in the respects tested, much better than those of composition A previously used. Owing to its price, composition B is hardly a possibility in such applications as the one indicated by way of intro- duction, whereas a surface coating based on the inventive composition is advantageously used in such contexts, being as it is much less expensive.

Further powder compositions according to the inven¬ tion are indicated in the Table below, which states the contents of each composition in per cent by weight, the balance being Fe.

Com . C Cr Mo Ni Mu Si O N

The hardness (HV 20) of surface coatings produced from these powder compositions by the PTA process is indicated in the Table below.

Comp. 20°C 300°C 500°C 700 ^C C 800°C

D

E

F

G H

I

J

K

L

Evidently, the hardness values in the above Table and the hardness values of powder composition C are comparable. As to the coefficient of thermal expansion and the oxidation resistance, surface coatings based on powder compositions D-L behave in essentially the same way as the surface coating based on powder composition C.

The following may be observed about the effect the different alloying elements have on the properties of the finished surface coating. Thus, chromium increases the resistance to oxidation, pitting, crevice corrosion and stress corrosion. It also increases wearing strength.

Nickel reduces the growth rate of cracks starting from pittings. Further, it increases the resistance to chloride stress corrosion, stabilises austenite, increases ductility and impact resistance, and also increases the corrosion resistance in neutral chloride solutions and weakly oxidising acids. When the composition contains more than 10% by weight of nickel, the resistance to stress corrosion also increases in austenitic stainless steel.

Carbon is an austenite stabiliser and carbide former and increases the wearing strength and the hardness of the finished surface coating.

Manganese is an austenite stabiliser and further binds sulphur to MnS.

Silicon improves the resistance to stress corrosion. Molybdenum increases the resistance to pitting and crevice corrosion in acid and neutral chloride solutions. Nitrogen improves the resistance to pitting and cre- vice corrosion in austenitic stainless steel. Also, it increases the strength and stabilises the austenite.