On the thermo-mechanical events during friction surfacing of high speed steels

doi:10.1016/S0257-8972(01)01129-X

Ž.

Surface and Coatings Technology 141 2001 34᎐ 39

On the thermo-mechanical events during friction surfacing of

high speed steels

G.M. Bedford

a,

U

, V.I. Vitanov

b

, I.I. Voutchkov

c

a

Department of Mechanical and Manufacturing Engineering, Uni¨ersity of Portsmouth, Anglesea Road, Anglesea Building,

Portsmouth PO1-3DJ, UK

b

Building 30, School of Industrial and Manufacturing Sciences, Cranﬁeld Uni¨ersity, Cranﬁeld, Bedfordshire MK43-OAL, UK

c

CEDC, School of Engineering Sciences, Uni¨ersity of Southampton, Highﬁeld, Southampton SO17-1BJ, UK

Received 31 July 2000; accepted in revised form 1 March 2001

Abstract

This paper is concerned with the friction surfacing of high-speed steels, BM2, BT15 and ASP30 onto plain carbon steel plate.

The events that the matrix and carbides experience as the coating material pass from the coating rod to the substrate, in forming

the coating, is described. The coating is observed to harden automatically within a few seconds of being deposited onto the cold

substrate. This autohardening is observed to be an inherent feature of the friction surfacing process and the only post-coating

heat treatment required is tempering, as with traditionally hardened high-speed steels. The mechanism of autohardening is

Keywords: Friction surfacing; Hardfacing; High speed steels

1. Introduction

High-speed steels were introduced at the beginning

of this century. The process used to produce them

involves casting and rolling which results in coarse,

inhomogeneous arrays of carbides that do not bring out

the best in this class of steels. In the second half of the

century, however, there have been considerable efforts

devoted to other methods of processing high speed

steels to produce ﬁne, homogeneous arrays of carbides

embedded in a high alloy martensitic matrix. The ASP

family of high-speed steels was developed in Sweden in

the 1960’s and is now widely used. As Fig. 1a,b prompts,

they possess the following properties in comparison to

conventional high speed steels:

U

Corresponding author.

䢇

a higher hardness, higher content of elements

forming carbides;

䢇

a higher toughness, the material is free from car-

bide segregation; and

䢇

isostropic properties, the material exposes a struc-

ture with an even distribution of carbide particles in

the matrix phase.

ASP steels have solved the problem of the heteroge-

neous distribution of carbides by using a powder route

instead of the traditional casting and rolling route,

however, they still require the traditional heat treat-

ment. More recently, other suppliers have developed

high-speed steels, also using powder routes, in which

ﬁne homogeneous arrays of carbides are generated, e.g.

Bohler, Austria.

¨

In the past decade, a coating method for high speed

steels has been developed, Fig. 2a,b in which as-

Ž.

PII: S 0 2 5 7 - 8 9 7 2 0 1 01129-X

()

G.M. Bedford et al. r Surface and Coatings Technology 141 2001 34᎐ 39 35

Ž.

Fig. 1. Conventional high-speed steel a and ASP family high-speed

Ž.

steel b .

deposited high speed steel not only has the ﬁne dis-

tribution of carbides achieved by powder routes, such

as ASP, but is also in the fully hardened state requiring

only tempering in the manner used for traditionally

hardened steels. Details of the friction surfacing process

wx

can be found elsewhere 1᎐6.

The work in this paper describes the coating process

and the use of microscopy, heat treatment and hard-

ness testing to study the events that the material un-

dergoes as it moves from the coating mechtrode,

through the interfacial zone onto the substrate. The

effect of tempering on the hardness of the coating is

also determined. In this paper, ‘mechtrode’ is used as a

term for the involved coating rod, which is being con-

sumed during the process.

2. Experimental development and procedure

Experiments using a range of mechtrode diameters,

Ž. Ž.

Fig. 2. a Principle of friction surfacing and b plain view of coating

using a 5-mm diameter mechtrode.

Fig. 3. Tempering curves 2= 1 h of T15, M2 and ASP30.

from 5 to 32 mm, were carried out at Frictec Ltd on a

commercial machine and also on a research machine at

the University of Portsmouth. The friction surfacing

process parameters for three high-speed steels Table 1,

were optimised using an approach described elsewhere

wx

7 . Plain carbon steel plate was used for the substrates.

Heat treatment of samples was carried out with the

temperatures controlled to 1⬚C. Vickers hardness test-

ing was conducted on ground surfaces, cleaned using

emery paper after each tempering treatment. The met-

allography was carried out on both plan and cross-

sections in both as polished and etched conditions. The

study of the morphology and composition of the car-

bides at different stages of the progress from coating

rod to coating is currently in its early stages and will be

published in the near future.

3. Results

Fig. 3 shows the results of tempering the different

high-speed steels in the upper temperature range

350᎐600⬚C. The hardness proﬁle of the heat affected

zones was also measured and these are shown as Fig. 4.

Fig. 5 compares a hardness proﬁle of a friction surfaced

deposit with an arc welded deposit and a microstruc-

Ž

ture identifying the carbides in the BM2 coating see

.

Fig. 6 .

Table 1

BT15, BM2 and AMT 30 compositions with forging and austenitising temperatures

High speed C W Mo Cr V Co Forging Austenitising

Ž. Ž.

steels range EC range ⬚C

BT15 1.4᎐ 1.6 12᎐ 13 0᎐ 1.0 4.25᎐ 5.0 4.75᎐ 5.25 4.5᎐ 5.5 982᎐ 1177 1204᎐ 1260

BM2 0.8᎐ 0.9 6.0᎐ 6.75 4.75᎐ 5.5 3.75᎐ 4.5 1.75᎐ 2.05 0᎐ 0.6 927᎐ 1149 1191᎐ 1232

AMT 30 1.28 6.4 5.0 4.2 3.1 8.5 Not known 1000᎐1180

()

G.M. Bedford et al. r Surface and Coatings Technology 141 2001 34᎐ 3936

Fig. 4. Heat affected zones of M2 and ASP30.

4. Discussion

An inherent feature of friction surfacing is the au-

tohardening of high-speed steels such that only temper-

ing remains to be carried out after coating. The tem-

pering curves Fig. 3, show the familiar secondary hard-

ening shape before rapidly losing hardness above 550⬚C,

in common with traditionally hardened and tempered

high-speed steels. A further important feature of fric-

tion surfacing is the consistency of the autohardening

as illustrated in Fig. 4. The hardness has almost an

order of magnitude improvement in consistency over

the fusion welding based coating process and this is

important in ensuring that properties are constant along

the length. This is particularly valuable in products

such as machine knives used in the processing and

packaging industries when cutting life and scheduled

tooling changes are an important cost in the process.

The heat affected zone in the substrate is localised

Ž.

and can be seen Fig. 4 to be less than 0.5 mm into the

substrate. This is signiﬁcantly different from many other

surfacing processes based on fusion and laser processes

in which the whole component reaches elevated tem-

peratures. In the case of friction surfacing, only lo-

calised heating occurs and the overall component,

although becoming ‘hot’, does not reach elevated tem-

peratures and hence affects the properties of the sub-

strate deleteriously.

The coatingrsubstrate bond is essentially formed by

a diffusion bonding mechanism. The micrograph in Fig.

6 indicates the different types of carbides that form in

BM2. The mechanism of transfer involves the material

being removed from the mechtrode and rolling over

onto the substrate.

In considering the thermo-mechanical events during

friction surfacing Fig. 7a,b,c serves to show how coating

material transfers from mechtrode to substrate. Fig. 7a

is a cross-section of the process showing the principle

of the coating system and Fig. 7c is a detail of Fig. 7a

and shows how the rubbing interface separates

mechtrode material from the material that forms the

coating. As the substrate draws across the face of the

rotating mechtrode the material at the rubbing inter-

face will either go towards developing the ﬂash or will

form the coating. The coating immediately beneath the

mechtrode will experience temperatures in the region

of 1020⬚C until forced into contact with the cold sub-

strate when heat transfer occurs across the

coatingrsubstrate boundary and the bond is formed.

The diffusion bond is a result of the force applied to

the mechtrode together with the heat generated at the

rubbing interface.

The depth of the HAZ is a function of the tempera-

ture at the coatingrsubstrate interface and the time at

temperature, i.e substrate speed. The cold substrate

causes rapid cooling resulting in the transformation of

Ž.

Fig. 5. Hardness variance proﬁle along 250-mm length of high speed steel deposit in the tempered condition of a fusion welding based surfacing

Ž.

process "4 Rc and b friction surfaced "1 Rc.

()

G.M. Bedford et al. r Surface and Coatings Technology 141 2001 34᎐ 39 37

Fig. 6. Microstructure of M2 coating, consisting of tempered marten-

Ž. Ž.

site and carbides: M C white ; and MC gray .

6

austenite to martensite. As can be seen from the pho-

tograph Fig. 7b, the whole coating is cooled to the

temperature of the substrate within a few seconds of

being deposited, which indicates a rate of cooling in

excess of 400⬚Crs. The effect of the friction surfacing

process is not only to take the carbides into solution,

but also to cause some mechanical effects, possibly

fracture of the carbides, before being cooled rapidly. A

study of austenitic 316 stainless steel, indicated that a

very ﬁne grained austenite structure is retained at

room temperature and this suggests that it is likely the

austenite in high-speed steels will be reﬁned due to the

thermomechanical effects of the rubbing interface and

that the martensite will be formed from ﬁne austenite

and, hence become ﬁne martensite, which will have

optimum properties following tempering.

In terms of the austenitising kinetics of the friction

surfacing process it is the mechtrode diameter that

plays a signiﬁcant role. Fig. 8 shows a plan view of the

coating and the elliptical area of the actual mechtrode

Ž.

Fig. 7. a, b and c Thermomechanical events during friction surfacing transfer of coating material to substrate.

Fig. 8. Austenitising times of high speed steel as a function of mechtrode diameter and substrate speed.

()

G.M. Bedford et al. r Surface and Coatings Technology 141 2001 34᎐ 3938

Ž.Ž. Ž .

Fig. 9. Time spent at elevated temperatures in austenitizing region as a function of traverse speed Vx a radial positions d.c. for a 32-mm

Ž. Ž .

diameter mechtrode and b time spent by central region of coating for given mechtrode diameters Md .

rubbing region. The graph of temperature vs. time in

Fig. 9a,b shows that along the diameter of the

mechtrodes, say 32 and 10 mm, moving at 1 mm per

second, the region under the mechtrode experiences,

32 and 10 s of maximum temperature, respectively,

before cooling. Regions at radial distances have pro-

portionally less time at maximum i.e. austenitising tem-

perature. Therefore, it may be expected that there

could be some differences across the coating. It is

expected that such an effect would likely become more

evident at larger diameters, say 18 mm and above.

Experiments are currently being conducted to study

this aspect. Fig. 9b indicates the given austenitising

times for given diameters at various substrate speeds.

The conventional thermal cycle for high-speed steels

is shown as Fig. 10. The thermal cycle for friction

surfacing shows a rapid increase in temperature due to

rapid development of the rubbing interface tempera-

ture and the hot working occurring at the rubbing

interface. The time spent at the elevated temperature

is a function of the mechtrode diameter. The coating is

rapidly cooled immediately it leaves the rubbing inter-

face and this leads to the autohardening of the coating.

A1q1 h tempering treatment is then applied in the

traditional manner.

5. Conclusions

1. An inherent feature of friction surfacing is the

occurrence of autohardening of high-speed steels

such that only a post-coating tempering treatment

remains to be carried out.

Fig. 10. Traditional heat treatment cycle of high speed steels and friction surfacing thermal cycle followed by traditional tempering.

On the thermo-mechanical events during friction surfacing of high speed steels

Summary (1 min read)

1. Introduction

2. Experimental development and procedure

3. Results

4. Discussion

Figures (6)

Citations

Cites background or methods from "On the thermo-mechanical events dur..."

Cites methods from "On the thermo-mechanical events dur..."

References

Related Papers (5)