M02FeB2 Complex Boride­Base Hard Alloy Wear Parts

The hard-working Toyo Kohanresearchers. looking at the problemsof wear and corrosion in machineparts for extrusion and injectionmoulding, have devised a new seriesof hard alloys based on a complexmolybdenum-iron boride. Thecermet-type alloy presented by M.Komai and his colleagues at PM90consists of particles of a boride-basedceramic, similar to M02FeB2• with aheat -treatable ferrous bindercontaining Cr. Ni, Mo. and otherelements.Table 1 lists grades. physicalproperties and applications of thenew hard alloys, with ferrous binderscharacterisedas ferritic, martensiticor austenitic depending on theircompositions. Hardness andtransverse rupture strength valuesrange from 80 to 92HRA and from1000 to 2600MPa respectively. Theseare comparable with those of sintered

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carbides. but with densities lyingbetween those of wc-and TiC-basedmaterials.Injection moulding techniques arenow applied to a wide variety ofmaterials, not only plastics but alsocomposites. metal powders, ceramicsand even carbide-based hardmetals.Fillers such as glass. carbon andceramic fibres add to the problem ofrapid wear of machine parts.In tests based on abrasion andcorrosion, the new group of alloysgave several times the working life ofthe steels traditionally used in thisapplication. There are four distinctseries, V. C. H and M. with increasingCr and Ni and correspondinglyincreasing resistance to corrosion andoxidation. In each series. hardnessand TRS values change with boroncontent. which decreases withincreasing grade number.The new hard alloys are already being

used. not only for can-making toolsand hot copper extrusion dies but alsofor injection moulding machine partssuch as screws. ring valves and barrels(Fig.2). Standard grade for this latterpurpose is V50. The new alloystypically give several times theoperating life of such conventionalalloys as nitrided and PM steels.though shorter than expected timesmay sometimes occur due to the widevariety of injection mouldedmaterials.The paper reported investigationsinto the effects of changing carboncontents on structure and properties(see Fig. 1). In addition. corrosioncharacteristics of a number of the newalloys were determined. For thecarbon-variation experiments, alloyswere prepared from mixed andball-milled powders of Fe-l1.6B­14.SCr alloy, pure Mo, pure Cr.carbonyl nickel, carbonyl iron and

,

~ 2.40 o--oTRS 88.0s .·--a Hardness

S

0'"c .......~ 2.20 86.0,--; ?0.2 2.00 8.tO

~ :l

~ 1.00 82.00 0.05 0.10 0.15

Residual carbon content (wt %)

FIG. 1 Transverse rupturestrength and hardness of themodelhard alloys as a function ojresidual carbon content (paper byM Komai et al, presented at PM90,London]

FIG.2 Various injection mouldingmachine parts composed ofMo2FeB2 complex boride basenew hard alloys (paper by MKomai et aI, presented at PM90,London]

graphite. The compacts werevacuum-sintered for 20 minutes at1290xC. Residual carbon wasdetermined by a Leco automaticanalyser. microstructure investigatedby SEM. TEM and X-ray diffraction,and physical and mechanicalproperties by standard procedures.Wear tests were carried out on anapparatus similar to that of ASTMB611-76 (figure 2).Quoted results indicate that theferrous binder of the model alloyschanges from ferritic to martensiticwith increasing residual carboncontent. The best combination ofTRSand hardness of the model alloys wasobtained when the martensitic binderand very little austenite and MaC arepresent with the M02FeB2 complexboride. This was achieved witharound 0.1% residual carbon.Abrasion test results were also betterwith this variant than with ferriticand austenitic binders.Surprisingly. the authors report noattempts to modify or optimise thebinder structures by heat treatment.In the much older parallel series ofcarbides with ferrous binders(Ferro-Tic and Ferro-Titanit],component manufacture is mademore economical and propertiesimproved by appropriate heattreatments after sintering.

KJB.

MPR July/August 1990