FRACTURE FEATURES OF SINTERED LOW-ALLOY STEEL PRODUCED BY METAL INJECTION MOLDING

S.V.Zavadiuk 1,
 
P.I.Loboda 2,
 
T.O.Soloviova 2*,
 
I.Iu.Trosnikova 2,
 
O.P.Karasevska 3
 

1 Science Industrial Association FORT of the Ministry of Internal Affairs of Ukraine, 27, 600-letiya str., Vinnitsa, 21027, Ukraine
2 National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, 37, Prosp. Peremohy, Kiev, 03056, Ukraine
3 G.V. Kurdyumov Institute for Metal Physics of the NAS of Ukraine, 36 Academician Vernadsky Blvd., Kyiv, 03142, Ukraine
tsolov@iff.kpi.ua

Powder Metallurgy - Kiev: Frantsevich Institute for Problems of Materials Science NASU, 2020, #11/12
http://www.materials.kiev.ua/article/3151

Abstract

In the manufacture of sintered steels by metal injection molding (MIM), typical microstructural defects, such as pores and pore agglomeration, phase structure heterogeneity, and boundaries between different phases, are hard to avoid. Such heterogeneities always cause crack origination, growth, and propagation when sintered materials are subjected to mechanical loads. The crack propagation path and cracking resistance are associated with complex heterogeneous structure including ferrite, cementite, martensite, pores, and weak interfaces. With increasing holding time in sintering, metal grains grow rapidly, leading to brittle fracture of the samples. The subsequent heat treatment substantially decreases the grain size and change brittle fracture to ductile one. Multicycle sintering of the Catamold 8740 low-alloy steel greatly increases the impact strength of notched samples (from 7.55 to 13.95 J/cm2). Greater density of the samples and fewer stress concentrators favorably influence the material’s capability to withstand impact loads. Thus when density of the billets following six sintering cycles increases by 2.5%, their impact strength becomes 1.8 times higher. With a greater number of sintering cycles, the ductile dimples become significantly larger, while the increase in shock impact and density of the sintered material gradually slows down. The grain size substantially increases (in turn, suppressing pore healing) and density of the samples becomes greater over the total sintering time. X-ray diffraction and spectral analysis revealed additional phases after sintering and heat treatment. Additional fine-crystalline carbide and oxide phases become more distinguished with higher increase in the sintering temperature and use of heat treatment. Brittle inclusions along with residual porosity present in sintered steel decrease the dynamic properties of the material.


FRACTURE SURFACE, IMPACT STRENGTH, POWDER INJECTION MOLDING, SINTERING