组织形态对球铁曲轴疲劳特性的影响

用两根以不同工艺铸造的4JB1发动机球铁曲轴,来研究组织形态对其疲劳特性的影响。一种工艺为铸态 正火处理(以下简称正火态);另一种是铁型覆砂铸造(以下简称铸态)。两者都经机加工、氮化处理后进行弯曲疲劳试验,并从毛坯取样做力学性能和冲击试验。结果表明:尽管两者的力学性能相近,但铸态曲轴的弯曲疲劳极限远高于正火态曲轴。这是由于铸态曲轴具有细小的牛眼状石墨形态 珠光体组织,所以裂纹起裂功和扩展功都较高,在疲劳中当石墨处萌生裂纹后,包围石墨的铁素体可以使裂纹尖端钝化,有效阻止裂纹扩展;而正火态曲轴显示为球状石墨 珠光体组织,包围在石墨周围的珠光体强度较高,在与石墨交界处萌生裂纹和之后裂纹的扩展都较快,导致疲劳寿命较低。

The Effect of Structure Morphology on the Fatigue Properties of Spheroidal Graphite Cast Iron Crankshafts

Taking 2 spheroidal graphite cast iron crankshafts of 4JB1 engine with different casting processes as samples,the effect of structure morphology on their fatigue properties is studied.One process is normal casting followed by normalizing treatment(referred to as 'normalized' hereinafter) while another uses sand-lined metal mould casting(referred to as 'as cast' hereinafter).Both of them are machined and nitridized followed by bending fatigue test.The mechanical and impact properties tests are also conducted on testpieces taken from crankshaft blanks.The results show that their mechanical properties are close to each other,the bending fatigue limit of as-cast crankshaft,however,are much higher than that of normalized one.The reason is that the as-cast crankshaft has a bull's-eye-like structure(spheroidal graphite surrounded by a layer of ferrite) in pearlite matrix,so both its crack initiation work and propagation work are higher.When cracks are initiated in the graphite,the crack tips can be blunted and the crack propagation can be impeded efficiently by ferrite surrounding the graphite;While the normalized one is composed of spheroidal graphite and pearlite.The pearlite surrounding the graphite has higher strength,so both the crack initiation on the boundary of graphite and the subsequent crack growth are quicker,hence leading to a lower fatigue limit.