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FRACTURE BEHAVIOUR OF A HIGHLY ALLOYED HIGH SPEED STEEL
Author(s) -
Gomes M. A.,
Wronski A. S.,
Wright C. S.
Publication year - 1995
Publication title -
fatigue and fracture of engineering materials and structures
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.887
H-Index - 84
eISSN - 1460-2695
pISSN - 8756-758X
DOI - 10.1111/j.1460-2695.1995.tb00137.x
Subject(s) - materials science , microstructure , nucleation , fracture toughness , metallurgy , carbide , composite material , powder metallurgy , high speed steel , alloy , fracture (geology) , grain size , chemistry , organic chemistry
Abstract A high speed steel, processed by two powder metallurgy routes and heat treated to give a range of microstructures, was investigated in 4‐point bending at room temperature using smooth and precracked specimens. The finer microstructures were in the material from gas atomised powder which was hot isostatically‐pressed, commercial ASP60 alloy, while the coarser microstructures derived from laboratory vacuum sintering of water‐atomised powder. The resultant hardness values H v50 were in the rage 780 to 1050, prior autenite grain sizes, 5 to 25 pm and maximum carbide sizes, 6 to 32 μm. Only some of the uncracked samples exhibited macroscopic yielding, at stresses in the range 1.64 to 2.59 GPa; the finer microstructures being asSociated with the higher strengths. Macroscopic plastic deformation never exceeded 0.33%; fracture strengths were in the range 1.46 to 2.75 GPa. Fracture toughness, K lc , varied from 12 to 17 MPa√m in the H y50 range 920 to 800 for the directly sintered steel and only from 10 to 12 MPa√m in ASP60. The insensitivity of K 1c to macroscopic hardness in ASP60 is asSociated with the plastic zone size of 1.5 μm which approximates to the average carbide spacing. Nucleation and growth of natural, i.e. stress‐induced, microcracks in un notched specimens was studied by surface replica microscopy. Crack nucleation took place at stresses between 0.5 and 1.5 GPa, i.e. below those for yielding and for fracture, σ F , and was by debonding of inclusions (alumina and calcium‐alumino silicates) or cracking of carbides. In the coarsest microstructure monotonic stepwise subcritical crack growth was observed from stress levels of ∼1.3 GPa, i.e. ∼0.8 to 0.9σ F . Similarities to the behaviour of short fatigue cracks in metallic materials and the R‐curve behaviour of ceramics are referred to.