The abrasive wear resistance of the carbide groups 6 and 8 hardfacing microstructures—The effect of abradent type

Int. J. Pres. Ves. & Piping$8 (1994)321-334 © 1994ElsevierScienceLimited Printed in Northern Ireland. All rights reserved

0308-0161/94/$07.00 ELSEVIER

The abrasive wear resistance of the carbide groups 6 a n d 8 hardfacing microstructures--T h e effect of abradent type J. H. Bulloch Electricity Supply Board, Head Office, Dublin 2, Republic of Ireland

& J. L. Henderson Babcock Power International, Mossell Bay 6500, Cape Province, Republic of South Africa (Received 21 May 1993; accepted 5 June 1993)

This paper describes an investigation aimed at evaluating the effects of abrasive medium type, viz. silicon carbide and river sand, on the wear resistance properties of the Groups 6 and 8 carbidic hardfacing microstructures. It was observed that a low alloy material with a fine dispersion of M7C3 carbides in a lenticular martensite matrix yielded the most promising and economical iron-based wear resistant material for high stress abrasive wear conditions. The relative abrasion resistance of the low alloy materials is significantly dependent upon chemistry and especially upon the chromium:carbon and total carbide former additions:carbon ratios. The dendritic high alloy material wear properties were insensitive to bulk chemistry considerations, while the primary carbide microstructures exhibited wear properties which were highly dependent upon chemistry and microstructure. Finally, the best wear properties were shown by the F e - C r - C eutectic microstructures. However, from a practical viewpoint this microstructure was difficult to attain because of the stringent boundary chemistry conditions.

which affect wear are material hardness, load, speed, surface roughness and temperature. Other factors which affect the nature and extent of wear are vibration, the presence and nature of loose abrasives, environment type and contaminants. The Group 6 chromium carbide irons contain roughly 25-30% Cr with 2-6% C with minor additions of mainly manganese and silicon. The microstructures of these materials are usually large primary hexagonal chromium carbides in an austenitic or martensitic matrix containing smaller carbides. The typical matrix hardness of these materials varies with alloy content. The lower alloys have a hardness of the order of 500HV and higher alloys in the range 650850 HV. The hardness of a typical carbide is about 1500 HV. Excellent abrasion resistance is exhibited by

INTRODUCTION A general term for the wear produced by a cutting or ploughing action is abrasive wear, which can probably best be defined as wear due to the penetration and ploughing out action of material from a surface by another body) Basically, two situations exist for this type of wear: two-body and three-body abrasive wear. In the present study a three-body abrasive wear situation was addressed, specifically open, threebody high stress abrasion which occurs during the progressive fragmentation or grinding of the abrasive, 2 e.g. in guides for hydraulic rams in a steel mill or steel balls in a ball mill. Such a situation can represent a costly industrial overhead. The principal factors, apart from lubrication, 321

322

J. H. Bulloch, J. L. Henderson

these materials, the austenitic irons 3 more commonly being used where wear is caused by loose abrasives such as sandy soil. With a martensitic matrix, a limited amount of medium impact can be tolerated providing the surface is adequately supported, and hence these materials may be used in grinding or high stress abrasion situations. These materials are, however, brittle in comparison to the homogeneous alloy type hardsurfacing alloys. It is usual for these materials to 'relief-check' on cooling from the welding temperatures. Relief-check is the term used to describe small cracks propagating perpendicularly into the hard surfaced material. These cracks form at right angles to the direction of travel of the hardsurfacing weld bead. The difference in the coefficient of contraction (expansion) tends to create a tensile stress of the longitudinal axis of the weld bead. This contraction is counteracted by the base material; cracking of the surface layer then takes place, relieving the tensile stresses and minimising distortion. These cracks, however, are not detrimental to the surface layer and do not propagate into the base material. Typical applications for these materials are agricultural machinery, mine and quarry screens and chutes, sand-blasting plant and steel mill guides. For extreme abrasion resistance, the Group 7 tungsten carbide irons are probably the best of all the hardsurfacing materials. The hardness of the tungsten carbide particles is about 2000 HV and they are included in a martensitic matrix with a hardness varying from 600 to 1000 HV. As with the chromium carbide irons, these alloys are very brittle and can only be used in situations where impact is light. Typical areas of application are construction and farm machinery, where digging blades work in sandy soil. These materials can never be used in bearing applications or metal-to-metal contact. The surface of these materials is very rough as the tungsten carbide particles, being harder than the matrix, wear slowly and can protrude from the matrix surface. Any metal surface continually in contact with these carbides would very soon be worn away. The Group 8 complex carbide irons differ from the straight chromium or tungsten carbide irons, and the use of the term 'complex' indicates that more than one type of carbide is contained in the surfaced layer. Almost all the materials are based

on high chromium additions combined with additions of either molybdenum, niobium, vanadium, titanium, tungsten or manganese; all these additions being carbide formers. From the Group 8 tables it can be seen that a variety of materials with some or all of these elements in various combinations exist. The abrasion resistance properties of these alloys reside between those of the chromium types, i.e. better than chromium types but not quite as good as the tungsten types. These complex irons also have certain other advantages, for example that limited impact can be tolerated, especially with those materials containing niobium. Another advantage is good oxidation resistance and hot hardness to over 600°C. Such advantages obviously dictate application areas for these materials, namely hot wear applications such as screens, scrapers, pulverisers and sinter plant. The present paper attempts to describe an investigation which was conducted to assess the relative abrasion resistance of a number of Groups 6 and 8 carbide hardfacing microstructures and the influence of abradent type.

EXPERIMENTAL TECHNIQUES In practice the plate dimensions upon which the various hardfacings were deposited, by a submerged arc welding technique, were 2500 mm x 1200 mm x 10 mm thick. The various chemical compositions of (i) the dendrite and eutectic and (ii) the primary carbide depositions are listed in Tables 1 and 2 respectively. Various samples which were used to identify the hardness, microstructure and wear abrasion resistance were cut from each deposition. A detailed report of the welding parameters is given in Ref. 4. The wear samples were about 20 mm x 20 mm square. A steel rod about 13 mm in diameter and 20 mm in length was glued to a sample on the face opposite the weld surface; this enabled the test specimen to be fixed into the test machine. Initially a series of mild steel samples of varying surfaces area were tested in order to assess the influence of contact area. Each specimen was tested five times. The first two runs were 'running-in' tests while the last three were tested against a mild steel standard

323

Effect of abradent type on carbidic microstructures Table 1. Groups 6 aml 8 dendritic and eutectic deposit ¢hemkal ¢oatpmitioa (wt%) Sample no.

C

6 26 28 31 34 41 43 53 57

(2.80) (2.70) (3-30) (2.00) (2.40) (3.40)

Ni

Mn

Mo

Cr

Nb

0-19 0.30 0.08 0.09 0.13 0.15 0.08 0.19 0.07

1.80 0.39 0.12 0.85 1.34 0.41 0.18 0.17 1.13

-0.66 -1.13 0"07 6"5 --0"03

27-60 27.50 23.10 14.8 17.00 14.4 26.70 25.70 5.00

V

Al

Si

Ti

-0.78 -0.03 0.05 ---0"01

0.02 <0-01 0.01 0.08 0.05 <0"01 0"01 0.01 0.10

0.86 0.94 0.88 0.94 1.33 2.12 1.09 1.12 1.25

0"05

Table 2. Groups 6 md 8 primary carbides deposit chemical composition (wt%) Sample no.

C

Ni

Mn

Mo

Cr

10 13

(4.10) (3.50)

(3.60)

19 21 22 27 29 32 42 56 61 62 63 64

(3.90) (5.20) (3.40) (4.10)

1.04 1.20 1.16 1.18 0.68 0.47

--1.90

18

0.16 0.16 0-53 0.05 0.25 0.24 0.16 0-18 0.14 0.12 0.16 0.14 (0.16)

0.45 1.80 1.42 0.17 -0-24 2-75

0.12

2-45

7

8

(4.40) (4.30) (2.60) (3.60) (3.10) (3.40)

V

AI

Si

18-40 23.40 19.50 29.70 28.60 23.5

--0.18 -0.04 --

0.04 0.02 0.03 0.03 0.02 0.031

1-42 1.38 1.07 0.73 1.14 1.14

---0.48 0.03 ---

26.00 27.60 24.30 9.70 -18.00 21.75

----0.09 ---

0.17 0.02 0.05 0.11 <0.01 0.21 0.18

2.00 0.86 1.07 0.25

1.60

17.60

0.20

--

1-62

1.00

which enabled the calculation of the Relative Abrasion Resistance, RAR. Two types of abradent materials, river sand and silicon carbide, were used and after every t e s t t h e w e i g h t loss w a s r e c o r d e d . T h e w e a r f a c i l i t y u t i l i s e d in t h e p r e s e n t i n v e s t i g a t i o n w a s o f the 'Abex wet sand' type which has been proved to simulate a three-body high stress abrasion

Nb

~

~

FAIC N

HARD

~

Ti

0-36 0.28

GLUEDTOSAMPLE

BASEMETAL

~

LOAD

W A T E RAND ABRASE iI MED U IM

41~

~

DR I ECTO IN OFTRAVEL

Fig. 2. Prepared specimen for testing on Abex machine. situation. Schematic details of the Abex test facility a n d t h e w e a r t e s t s p e c i m e n a r e s h o w n in Figs 1 a n d 2 r e s p e c t i v e l y .

EXPERIMENTAL

\ C O P P E R TRACK Fig. I. Schematic of Abex abrasive test machine.

RESULTS

Basically the microstructure of the hardfacing materials within Groups 6 and 8 can be subdivided into three separate microstructural c a t e g o r i e s , viz. p r i m a r y a u s t e n i t e w i t h a n i s l a n d

324

J. H. Bulloch, J. L. Henderson

Fig. 3. Primary austenite with an interdendritic eutectic (original magnification × 1000).

Fig. 4. Complete carbide eutectic (original magnification x 1000).

carbon level. From this figure it is evident that both the large and small volume fractions of primary carbide microstructures exhibited a significant level of scatter and no significant influence of carbon level was observed. However, in the case of the dendritic microstructure the R A R exhibited a decrease with increasing carbon level. The solitary eutectic microstructure data point exhibited the highest R A R value which was about 25% higher than those shown by the dendritic microstructures. From Fig. 7, which shows the relative abrasion resistance ( R A R ) data as a function of chromium level, it is evident that both the small and large carbide volume fraction structures showed significant decreases in R A R with chromium level, while the dendritic microstructure exhibited R A R values which were insensitive to chromium over the range 5 - 2 7 % Cr. Again the eutectic structure exhibited the highest R A R value. The abrasive wear resistance of the various microstructures as a fraction of total carbideforming alloying (TCFA) elements is shown in Fig. 8. From this figure both the primary carbide structures showed a marked decrease in R A R with percent T C F A elements while the dendritic structure exhibited little change over the range 6 to about 27%. A similar trend in R A R values is also evident in Fig. 9 which shows the influence of the ratio of chromium to carbon contents on the R A R data. Figure 10 illustrates that both primary carbide structures exhibited a decrease in R A R with an increase in the ratio of T C F A elements to carbon content while the dendritic microstructure was insensitive to this parameter. River sand R A R data

Fig. 5. Primary carbides within a eutectic (original magnification x 1000). eutectic (Fig. 3), complete carbide eutectic (Fig. 4) and primary carbides within a carbidic eutectic (Fig. 5). Silicon carbide R A R data The abrasion resistance of the various microstructure types is shown in Fig. 6 as a function of

The influence of carbon level on the relative abrasion resistance ( R A R ) characteristics of the various Groups 6 and 8 microstructures is shown in Fig. 11. From this particular figure it is evident that the primary carbide microstructures, low and high carbide volume fractions, exhibited significant data scatter and no apparent trend with carbon content. In the case of the dendrite microstructure it would appear that the R A R is significantly increased with carbon level (bearing in mind that only two data points exist). Figure 12 shows the R A R results as a function of chromium content. It is evident that both the high volume fraction carbide and dendritic

Effect of abradent type on

325

carbidic microstructures

[]

RELATIVE ABRASION RESISTANCE

O

" " - --.,.,.. 2

0

-

<>

o

0

•O •

LEGEND

O

Eutectic 0

Dendritic

0

Large Volume Fractlon Carbides

•

Small Volume F r a c t i o n Carbides

1

I

I

0.1

I

0.2

I

O.]

0.5

0.4

% CARBON

Fig. 6. Effect of carbon content on wear resistance for a silicon carbide abradent.

decreased with increasing percent TCFA elements. Figure 14 shows the influence of the chromium to carbon ratio on the R A R results for the three separate microstructures. It is evident that the trends are similar to those shown in Fig. 12. Finally, the effects of the percent TCFA elements to carbon level ratio on the R A R characteristics

microstructures exhibited around a 30% and 20% decrease respectively with increased chromium level. The results for the low volume fraction primary carbide structure were difficult to assess. The effect of the percent TCFA elements on the R A R is illustrated in Fig. 13 from which it is evident that all three microstructures exhibited significant effects, in as much as the R A R

0 0

0

RELATIVE ABRASION RESISTANCE

LEGEND m

Eutectic

0

Dendritic

0

Large Volume F r a c t i o n Carbides

•

Smell Volume F r a c t i o n Carbides

I I0

I

I

20

)0

% CHROMIUM

Fig. 7. Effect of c h r o m i u m content on wear resistance for a silicon carbide abradent.

326

J. H. Bulloch, J. L. Henderson

0

0

2 []

¢

------L_

°

Relative Abrasion Resistance

LEOEND m

i

Eutectic

o

Dendritic

0

Large Volume Fraction Carbides

Small Volume Fraction Carbides

I

,,.I

I0

20

,,,

1

O 30

% Total Carblde Forming Addltions

Fig. 8. Effect of carbide-forming elements on wear resistance for a silicon carbide abradent. Influence o f abradent type

of the various microstructures are given in Fig. 15. In all cases increasing this ratio decreased the R A R for the microstructures involved, indicating that for this particular abradent type, viz. river sand, low chromium and high carbon contents appear beneficial to the wear resistance properties.

From Fig. 16 it can be seen that in the case of the dendrite microstructure the R A R decreased and increased with carbon level for the silicon carbide and river sand abradent types, respectively. The high and low volume fraction primary carbide

[3 n

a--

0 0 ~ ' " ~

RELATIVE ABRASION RESISTANCE

LEGEND Eutectic

.... 0

0

Dendritic

0

Large Volume Fraction Carbldes

•

5mall Volume Fraction Carbides

J

I

L

I

3

6

9

]2

% CHROMIUR CARBON

l~g. 9. Effect of Cr: C ratio on wear resistance for a silicon carbide abradent.

Effect of abradent type on carbidic microstructures

327

O O~ []

O

-O

RELATIVE ABRASION RESISTANCE

LEOENO m

Eutectic

[]

Dendritic

O

Large Volume Fraction Carbldes

•

Small Volume Fraction Carbides

0

I

I

I

I

2

4

6

8

I 10

12

% TCFA % Carbon

Fig. 10. Effect of total carbide-forming additions ( T C F A ) to carbon content ratio on wear resistance for a silicon carbide abradent.

microstructures, however, exhibited no obvious trend with carbon contents and showed significant data scatter. The effects of chromium level on the R A R values for the two abrasive media are illustrated

in Fig. 17. It is evident that the dendrite structure exhibited little effect of percent Cr for the silicon carbide abradent, while with the river sand the R A R results decreased with percent Cr. The high volume fraction primary carbide microstructure

2.0

[]

0

0 Relative Abrasion Resistance

0

1.5

LEGEND [] 0 •

Dendritic Large Volume Fraction Carbides Small Volume Fraction Carbides

I

I

0.2

0.3

I 0.4

,~ CARBON

Fig. 11. Effect of carbon content on wear resistance for a river sand abradent.

0.5

328

J. H. Bulloch, J. L. Henderson 2.0

<> 0

RELATIVE ABRASION RESISTANCE

0

1.5

LEGEND 0

Dendritic

0

Large Volume Fraction Carbides

•

S m a l l Volume Fractlon Carbides

I

I 20

10

30

% CHROMIUM

Fig. 12. Effect of chromium content on wear resistance for a fiver sand abradent. R A R characteristics showed a decrease with percent Cr for both abradent media, while the small volume fraction carbide structure exhibited little effect of abradent type. The trends observed in the percent TCFA

elements relationship with R A R were similar to those shown in Fig. 18 except that the river sand data exhibited a larger decrease in R A R than the silicon carbide data for the large volume fraction carbide microstructure (see Fig. 18). Indeed the

2.0

0

0

(>.~. RELATIVE ABRASION RESISTANCE

0

1.5

LEGEND

I

0

Dendrztlc

0

Large Volume Fractlon Carbldes

•

5mall Volume Fractlon Carbides

I

I

1

10

20

30

% TOTAL CARBIDE FORMING ELEMENTS

Fig. 13. Effect of carbide-forming elements on wear resistance for a fiver sand abradent.

329

Effect of abradent type on carbidic microstructures 2.0 ¸

~

RELATIVE ABRASION RESISTANCE

<>

\\ D

1.5

LEGEND []

Dendrite

0

Large Volume Fraction Carbide

1

Small Volume Fraction C a r b i d e

I 2

0

I

I

I

4

6

8

I

10

% Cr %C

Fig. 14. Effect of Cr: C ratio on wear resistance for a river sand abradent.

same description could be applied to the ratios of chromium to carbon levels and of percent TCFA elements to percent carbon (see Figs 19 and 20 respectively).

rendered any clear trends difficult to assess. However, in the case of the dendrite microstructure, with the exception of the percent carbon, the RAR values for the silicon carbide medium were insensitive to all other chemical parameters while the river sand results exhibited consistent significant decreases in R A R with chemical considerations. Also in all cases the R A R values for the silicon carbide abradent were higher than those for the primary carbide microstructures, while the

DISCUSSION From the foregoing section it is evident that the river sand data exhibited significant scatter which 2.0

i\

RELATIVE ABRASION RESISTANCE

0

[] Oendrlte O

Large Volume Fraction Carbide

Small Volume Fraction Carbide

1

I

I

|

2

4

6

I 8

10

TCFA ~U F i g . 15.

Effect of total carbide-forming additions ( T C F A )

to carbon content ratio on wear resistance for a fiver sand abradent.

330

J. H. BuUoch, J. L. Henderson

O

aDO •

/[]

O O

RELATIVE ABRASION RESISTANCE

o

LEGEND

[]

Dendritic, Silicon Carbide Dendritic, River Sand

o

Lamge Amount Of Carbides, Silicon Carbide Large Amount of Carbides, River Sand

I 0.i

I

I

I

0.2

0.3

0.4

0.5

% CARBON

Fig. 16. Comparison between wear resistance for different abradents as a function of carbon content. parameters. A l s o they exhibited R A R values which were slightly lower than those recorded by the dendritic microstructures. The observation that the R A R data for the river sand abrasive m e d i u m were generally smaller than those for the silicon carbide

solitary eutectic microstructures data point exhibited an R A R value which was s o m e 2 5 - 5 0 % higher than for the other microstructures. Gen,.'ally, both primary carbide microstructures s h o w e d a decrease in R A R for both abradent media for all chemical or compositional

[]

[]

0 ¸

[]

[] RELATIVE ABRASION RESISTANCE

O\

LEGEND I-

0

Dendritic,

Silicon Carbide

•

Dendritic,

River Sand

0

Large Volume Fraction Carbides, SiIicon Carbide

@

Large Volume Fraction Carbides, River Sand

I 10

I 20

f 30

% CHROMIUM

Fig. 17. Comparison between wear resistance for different abradents as a function of chromium content.

Effect of abradent type on carbidic microstructures

[]

0

331

Q

,

2 0

$-- - - o .

RELATIVE ABRASION RESISIANCE

xxO \

\

\ •\ LEGEND O

D e n d r i t i c , S i l i c o n Carbide

•

D e n d r i t i c , River Sand

0

Large Volume Fraction Carbides, Silicon Carbide

•

Large Volume Fraction Carbides, River Sand

1

I

,I

10

20

I 30

TOTAL CARBIDE FORMING ELEMENIS

l~g. 18. Comparison between wear resistance for different abradents as a function of total carbide-forming elements.

abradent can be explained simply from experimental considerations. Basically the R A R represents a ratio between the weight loss from a mild steel and a hardfacing sample, and as mild steel is more resistant to fiver sand than a silicon carbide abrasive medium the river sand weight

loss ratio would be smaller than that for the silicon carbide abradent. Hence the lower RAR results for all samples in river sand. One factor that is evident from the study is the strong effect that microstructure had on the wear resistance of the present materials. Thus changes

Q 0

rl

0

RELATIVE ABRASION RESISTANCE

•,-,.. •

•\ LErJ~ND D

D e n d r i t i c , S i l i c o n Carbide

•

D e n d r i t i c , River Sand

0

Large Volume Fraction Carbid~

S i l i c o n Carbide

•

Large Volume Fraction Carbid~

River Sand

I

I

1

I

2

4

6

8

Cr ,~C

Fig. 19. Comparison between wear resistance for different abradents as a function of Cr: C ratio.

10

332

J. H. Bulloch, J. L. Henderson

0 0

o

O

-'-" 0 ~

•

0

RELATIVE ABRASION RESISTANCE

x •

k

\

LEGEND 0

Dendritic, 511icon Carbide

•

Oendrltic, River Sand

0

Large Volume Fractlon Carbide, Silicon Carbide

•

Largo Volume Fraction Carblde, River Sand

I

I

2

4

%ICFA ~C

1

I

I

6

8

i0

12

Fig. 20. Comparison between wear resistance for different abradents as a function of total carbide-forming additions (TCFA) to carbon content ratio.

such as through-thickness changes in microstructures can significantly affect wear properties. The present hardfacing microstructures under study, viz. the carbidic structure, exhibited microstructure differences from surface deposit through to the fusion line. Indeed all carbidic hardfacing deposits have a fusion line microstructure of primary austenite dendrites: see Fig. 21. From Fig. 22 it can be seen that, within the austenitic layer, a partial transformation of austenite to lenticular martensite has occurred. The range of carbide microstructures that can be present throughout the depth of any carbide type deposit is shown schematically in Fig. 23. If the vertical

Fig. 21. Primary austenite dendrites (original magnification x 1000).

Fig. 22. Austenite interface showing partial transformations of lenticular martensite (original magnification ×2400).

axis represents the chromium concentration and the interface of a welded sample, the base metal would be on the left-hand side and the thickness of the deposit would increase along the horizontal axis to some final thickness x. Line 1 represents a high-chromium electrode and line 2 a low-chromium electrode; they could equally well denote a high and low total alloy composition respectively. On the vertical axis are marked areas, starting from essentially 0% Cr at the interface, of austenitic through eutectic to primary carbide growth with increasing concentrations of chromium (or total alloy content).

Effect of abradent type on carbidic microstructures % Cr

Region ofI primary | carbides + | High-Cr electrode (i)

eutectic |

Region of eutectic growth

~

Region of austenitic growth i

Base metal

Q

Interface

Low-Cr

electrode (2)

I

Io Surface

~

Fig. 23. Schematiceffect of chromium gradient.

Upon solidification, a chromium (or alloy) gradient is set up in the deposit owing to dilution and segregation. Here these are shown as straight lines, but they may not necessary be so. It can be seen that line 1 with a higher chromium gradient passes quickly through the regions of austenitic and eutectic growth, ending in a surface microstructure of primary carbides in a eutectic matrix. Line 2, having a lower gradient, passes slowly through the region of austenite growth and, depending on composition, t h e material may not leave the zone of eutectic growth. Also from Fig. 23 it can be seen that the thickness of the layer of austenitic growth for line 1, shown here as A, is much thinner than in the corresponding region for line 2, shown as A 1. Distance B indicates the thickness of the layer of eutectic growth for line 1. The wear rates of any particular carbide deposit would change according to its microstructural changes throughout the depth of the deposit. As the deposit chemistry also changes throughout the depth of the deposit, the wear rate would then also vary according to chemical changes within each microstructural grouping. A previous study has reported the influence of abrasive medium on the Groups 2 and 3 martensite handfacing materials. 4 Hence a comparison between this group and the present

333

carbide microstructural materials would be instructive at this stage. The first and most noticeable comparison between the two groups is that the martensitic group has R A R values the same as, or higher than, those of the chromium carbide group in a silicon carbide abrasive. In a river sand medium, the R A R values are similar. This immediately suggests that the carbide materials are uneconomic in a high stress abrasive situation. For Groups 2 and 3 materials it was observed that an M7C3 type carbide in a lenticular martensitic matrix offered the best wear resistance, and the chemical requirements to achieve this were 0.4% C and 4% Cr. In the case of the silicon carbide, high percentages of C, Cr and TCFA were beneficial, but the ratio of percent Cr (or TCFA) to percent C should be about 10-12 for maximum wear resistance. In a river sand abrasive medium a high carbon level is beneficial while higher chromium levels are not, with better wear resistance being shown by other carbide-forming alloying additions. A coincidental ratio is, however, noted for C r : C and TCFA: C of about 12 as showing the optimum wear resistance in both silicon carbide and river sand abrasive media. For the Groups 6 and 8 materials, a dendritic microstructure offers the best achievable wear resistance with chemistry somehow appearing to have little effect in a silicon carbide medium. The eutectic structure obviously shows the best resistance, but in practice this structure would be difficult to achieve as it has tight boundary chemical conditions and would be significantly affected by heat input and dilution effects, etc. The large and small carbide volume fraction (CVF) microstructures appear inferior to the dendritic microstructures and are significantly affected by the chemical composition, viz. increasing the CVF decreased their R A R no matter what the carbide-forming element. The best R A R values for the large CVF microstructures are exhibited by a complex type, again indicating that an alloy containing carbide from other than chromium is beneficial in a river sand medium. Similarities therefore existed between the two general groups, i.e. the martensite and carbide materials. In a river sand medium, chromium was not the best alloy choice, while in a silicon carbide abrasive medium, large additions of any alloy addition, other than carbon, did not

334

J. H. Bulloch, J. L. Henderson

increase the wear resistance. It should also be noted that in both groups, the materials exhibited consistently good performances in a silicon carbide abrasive medium; this is not surprising since the alloys all fall in the same area of the ternary diagram, all solidifying from austenite plus M7C3 phase, resulting in a lenticular martensitic matrix with fine dispersions of M7C 3 carbides.

CONCLUSIONS

Under the given experimental conditions the following conclusions have been reached. 1. The use of a low alloy with M7C3 carbides in a lenticular martensitic matrix offered the best economical iron-based wear resistance in a high stress abrasive situation. 2. Generally the dendritic high alloy materials with dispersions of fine M7C3 carbides appeared to exhibit R A R values which were independent of chemistry considerations. 3. The R A R values of low alloy materials were d e p e n d e n t on composition and especially on the Cr: C and % T C F A : C ratios.

4. All the high and small carbide volume fraction materials exhibited R A R values which were significantly dependent upon chemistry and microstructure. 5. The material that exhibited the best abrasive wear resistance properties was the F e - C r - C eutectic. However, in practice this microstructure was difficult to achieve because of the tight boundary chemical considerations. 6. In the carbidic hardfacing deposits the changes in wear rate throughout the deposit depth were highly d e p e n d e n t on both chemistry and microstructure.

REFERENCES

1. Lansdown, A. R. & Price, A. L., Materials to Resist Wear. Pergamon Press, 1986. 2. Avery, H. S., Hardfacing Alloys. Technology Report no. C6-17-3, American Society of Metals, Metals Park, OH, 1978. 3. Cillett, H. W., Wear of Metal. ASTM Symposium, May 1967. 4. Henderson, J. L. & Bulloch, J. H., The abrasive wear resistance of the martensitic Group 2 and 3 hardfacing materials: The influence of abradent type. Int. J. Pres. Ves. & Piping, 57 (1994) 253-69.