WO1994011128A1

WO1994011128A1 - A composite roll

Info

Publication number: WO1994011128A1
Application number: PCT/AU1993/000591
Authority: WO
Inventors: Peter Robert Saxby
Original assignee: The Broken Hill Proprietary Company Limited
Priority date: 1992-11-18
Filing date: 1993-11-18
Publication date: 1994-05-26
Also published as: CN1096234A

Abstract

A cermet material, a composite roll comprising a core and a hardfacing of the cermet material, and a method of forming the composite roll are disclosed. The cermet material comprises particles of niobium carbide and an alloy, with the composition of the alloy comprising 30 to 45 wt. % iron based on the total weight of the alloy. When formed as the hardfacing on the core to form the composite roll, the structure of the alloy comprises secondary carbides dispersed in a matrix. The method of forming the composite roll comprises welding the cermet material onto the core.

Description

A COMPOSITE ROLL

The present invention relates to a cermet material suitable for use as a hardface of a composite roll, a composite roll comprising a metal core and a hardface of a cermet material, and a method of forming a hardface on a metal core.

The present invention relates particularly, although by- no means exclusively, to rolls used in the production of structural steel. Such rolls are usually cast from steel- based or iron-based materials.

The main requirements of rolls used in the production of structural steel are:

(a) abrasion resistance to minimise wear and localised damage to the roll surface;

(b) capability to wear uniformly so that it is possible to maintain close dimensional tolerances in the rolled product;

(c) resistance to thermal fatigue crack initiation and propagation;

(d) compressive and tensile strength to resist mechanical forces; and

(e) toughness.

The relative importance of these requirements varies with the stages in the rolling mill. For example, resistance to thermal fatigue crack initiation and propagation is a more important requirement in the roughing stand than in the finishing stand. Therefore, a roll which has a combination of properties which is acceptable for use in a finishing stand may not be acceptable for use in a roughing stand.

It has been proposed to use composite rolls comprising a metal core and a hardface of a cermet material in the rolling of structural steel. On a theoretical basis, cermet materials should be well suited to provide the requirements noted above, since by selecting the separate components of a cermet material to comprise a tough matrix and a hard carbide dispersion in the matrix and by adjusting the relative proportions of the components it should be possible to optimise the properties of the hardface to overcome the problem that some of the requirements noted above, such as abrasion resistance and toughness, are not necessarily compatible. However, on a practical basis, it has hitherto not been possible to produce a cermet material hardface on a metal core to form a roll which adequately complies with the requirements noted above. To a certain extent, this situation has been contributed to by difficulties applying a seemingly suitable cermet material to a metal core to form a hardface which is acceptable in terms of microstructure and bonding to the metal core. The microstructure considerations include the need to have a uniform dispersion of carbide particles and minimal local defects. A further factor which has made it difficult to use cermet materials is that it must be possible to machine the hardface to form a surface of acceptable quality.

An object of the present invention is to alleviate the disadvantages described in the preceding paragraphs.

According to the present invention there is provided a composite roll comprising, a core, and a hardfacing of a cermet material on the core, the cermet material comprising niobium carbides dispersed in an alloy, the composition of the alloy comprising 30 to 45 wt.% iron based on the total weight of the alloy, and the structure of the alloy comprising carbides dispersed in a matrix.

It has been found unexpectedly in experimental work on hardfacings of cermet materials on metal cores that the concentration of iron in the alloy affects significantly the distribution of the carbides (hereinafter referred to as "secondary carbides" in order to distinguish the carbides from the niobium carbides) in the alloy. Specifically, it has been found that in order to obtain a uniform dispersion of secondary carbides in the hardfacing of a cermet material, which is necessary in order to obtain uniform properties of the cermet material, the alloy should comprise 30 to 45 wt.% iron based on the total weight of the alloy.

It is preferred that the alloy comprises 32 to 43 wt.% iron based on the total weight of the alloy.

It is preferred particularly that the alloy comprises 35 to 40 wt.% iron based on the total weight of the alloy.

It is preferred that the cermet material comprises 10 to 60 vol.% niobium carbide and the balance being the alloy.

It is preferred particularly that the cermet material comprises 20 to 40 vol.% niobium carbide and the balance being the alloy.

It is more particularly preferred that the cermet material comprises 20 to 30 vol.% niobium carbide and the balance being the alloy.

Typically, the cermet material comprises 25 vol.% niobium carbide.

It is preferred that the alloy be an iron-cobalt based alloy or an iron-nickel based alloy. It is preferred particularly that the iron-cobalt based alloy and the iron-nickel based alloy comprise any one or more of the following elements.

(a) Chromium

Chromium forms a solid solution with cobalt and nickel.

In addition, chromium oxides form as a surface layer which provides excellent high temperature oxidation and corrosion resistance.

(b) Molybdenum, tungsten, niobium, tantalum, vanadium

These elements form carbides and as a consequence are added to increase the hardness and hot strength of the matrix. The elements also allow the matrix to age harden.

(c) Titanium, aluminium

These elements form intermetallic compounds which allow the matrix to age harden.

(d) Silicon

Silicon acts as a deoxidiser.

(e) Manganese

Manganese acts as a deoxidiser and a desulpheriser.

According to the present invention there is also provided a cermet material suitable for use as a hardfacing of a roll, the cermet material comprising niobium carbide and an alloy as described in the preceding paragraphs.

According to the present invention there is also provided a method of forming the composite roll described in the preceding paragraphs comprising, welding a mixture of particles of niobium carbide and the alloy and/or the constituents of the alloy onto the core.

It is preferred that the welding step be carried out by means of a plasma transferred arc hardfacing system.

It is preferred that the powder and plasma gases of the plasma transferred arc hardfacing system each comprise up to 10 vol.% hydrogen and the remainder argon.

The present invention is described further by way of example with reference to the accompanying photomicrographs in which:

Figure 1 is a low magnification photomicrograph (magnification 12.8) of a typical view of a hardfacing of a preferred cermet material in accordance with the present invention;

Figure 2 is a medium magnification photomicrograph (magnification 160) of the hardfacing; and

Figure 3 is a high magnification photomicrograph (magnification 500) of the hardfacing.

The hardfacing shown in the photomicrographs was formed by welding a cermet material comprising a mixture of 30 vol.% NbC, 50 vol.% Nistelle C, and 20 vol.% CΞAC high purity iron onto a steel substrate with a STELLITE STARWELD plasma transferred arc hardfacing system, model 600 torch. The hardfacing was laid in three runs. A mixture of argon and hydrogen was used for all gas streams. The chemistry of Nistelle C is set out below (amounts in wt.%) .

C Ni Mn Si Cr Mo Fe V 0.1 Bal 1.0 1.0 16.5 17.0 6.0 0.3 4.5

The photomicrographs show that the hardfacing comprised, a comparatively uniform distribution of primary carbides, i.e. niobium carbides, and a uniform distribution of secondary carbides in three distinct morphologies, namely:

(a) relatively large, blocks; (b) "Chinese-script"; and

(c) eutectoid;

in an iron-nickel based alloy.

The total concentration of iron in the alloy, having regard to the iron in the CEAC high purity iron and in the Nistelle C, was 37 wt.% based on the total weight of the alloy.

It was observed that there was no significant variation in morphology in the secondary carbides at interweld heat affected zones.

The hardness of the hardfacing was measured on polished and etched surfaces of the hardfacing. The following Vickers hardness values (using a 30 kg load) were obtained.

(a) First Weld Run = 335, 309, 330 and 318 mean = 323

(b) Second Weld Run = 350, 339, 341, 334, 328 and 326 mean = 336.3 (c) Third Weld Run = 327, 314,333 and 333 mean = 324.2

The above results indicate a near uniformity of weld run hardness and this finding, coupled with the observation that there was no significant morphological variation in secondary carbides at interweld heat affected zones, indicates that the hardfacing should exhibit a uniform wear response.

The foregoing example is illustrative of the unexpected impact of the concentration of iron in an alloy of a cermet material comprising niobium carbide particles dispersed in the alloy on the distribution of secondary carbides in the alloy phase of a hardfacing of. the cermet material. Specifically, as indicated above, it has been found that in order to obtain a uniform dispersion of secondary carbides in the hardfacing of such a cermet material, which is necessary in order to obtain uniform hardness properties of the hardfacing, the alloy should comprise 30 to 45 wt.% iron based on the total weight of the alloy.

More particularly it has been found that at concentratxons of iron less than 30 wt.% and greater than 45 wt.% in the alloy, based on the total weight of the alloy, there is a significant variation in the distribution of secondary carbides in the alloy phase that is produced when the cermet material is deposited as a hardfacing on a core. As a consequence, there is a significant variation of the hardness of the hardfacing which is unacceptable for a roll.

Claims

CLAIMS :

1. A composite roll comprising, a core, and a hardfacing of a cermet material on the core, the cermet material comprising niobium carbides dispersed in an alloy, the composition of the alloy comprising 30 to 45 wt.% iron based on the total weight of the alloy, and the structure of the alloy comprising carbides dispersed in a matrix.

2. The composite roll defined in claim 1, wherein the alloy comprises 32 to 43 wt.% iron based on the total weight of the alloy.

3. The composite roll defined in claim 2, wherein the alloy comprises 35 to 40 wt.% iron based on the total weight of the alloy.

4. The composite roll defined in any one of the preceding claims, wherein the cermet material comprises 10 to 60 vol.% niobium carbide and the balance being the alloy.

5. The composite roll defined in claim 4, wherein the cermet material comprises 20 to 40 vol.% niobium carbide and the balance being the alloy.

6. The composite roll defined in claim 5, wherein the cermet material comprises 20 to 30 vol.% niobium carbide and the balance being the alloy.

7. The composite roll defined in any one of the preceding claims, wherein the alloy comprises an iron-cobalt based alloy or an iron-nickel based alloy.

8. A cermet material suitable for use as a hardfacing of a roll, the cermet material comprising niobium carbides and an alloy, the composition of the alloy comprising 30 to 45 wt.% iron based on the total weight of the alloy, and the structure of the alloy comprising carbides dispersed in a matrix.

9. The composite roll defined in claim 8, wherein the alloy comprises 32 to 43 wt.% iron based on the total weight of the alloy.

10. The composite roll defined in claim 9 , wherein the alloy comprises 35 to 40 wt.% iron based on the total weight of the alloy.

11. The composite roll defined in any one of claims 8 to 10, wherein the cermet material comprises 10 to 60 vol.% niobium carbide and the balance being the alloy.

12. The composite roll defined in claim 11, wherein the cermet material comprises 20 to 40 vol.% niobium carbide and the balance being the alloy.

13. The composite roll defined in claim 12, wherein the cermet material comprises 20 to 30 vol.% niobium carbide and the balance being the alloy.

14. The composite roll defined. in any one of claims 8 to 13, wherein the alloy comprises an iron-cobalt based alloy or an iron-nickel based alloy.

15. A method of forming the composite roll defined in any one of claims 1 to 7 comprising, welding a mixture of particles of niobium carbide and the alloy and/or the constituents of the alloy onto the core.

16. The method defined in claim 15, wherein the welding step is carried out by means of a plasma transferred arc hardfacing system.

17. The method defined in claim 16, wherein the powder and plasma gases of the plasma transferred arc hardfacing system each comprise up to 10 vol.% hydrogen and the remainder argon.