Failure analysis of a clamping component from high performance machine

Introduction
High performance manufacturing machinery consists of components that possess superior resistance under severe friction and wear conditions combined with high strain rates. Fracture of critical machine components lead to high downtime periods and have serious implications in productivity, quality and maintenance cost.

Investigation
A fractured – in service – clamping component from heavy duty equipment is investigated. Chemical analysis showed that the material used corresponds to a high-chromium – high carbon tool steel, while its surface hardness reaches up to 62 Rockwell C (HRC) approximately. The microstructure consists of coarse primary complex Cr carbides dispersed in a tempered martensite matrix (Fig. 1). Such coarse primary carbides are hard and brittle microstructural constituents and constitute stress concentration sites and, on the other hand suffer from cracking and decohesion from the matrix under dynamic loading conditions. An appreciable high amount of non-metallic inclusions corresponding mainly to silicates and sulfides was also detected (see Fig. 2 and 3).

The specific steel grade and strength level corresponds most likely for high wear resistance applications while possesses low toughness under dynamic stressing or high strain rate conditions. Metallographic and fractographic investigation revealed the presence of a mixed mode fracture mechanism; crack propagation has been assisted through primary carbide network – while a limited amount of strain has been accommodated to the tempered martensite matrix which manifested a limited plasticity failure mode (see Figs. 4 and 5). Transgranular carbide facets with numerous secondary cracks (Fig. 4, 5a) and very fine nano-sized dimples grown around the secondary carbides are the principal microfractographic features addressing the proposed failure mechanism (Fig. 5b).

a)

b)

Fig. 1: Optical micrographs showing (a) the microstructure underneath the fracture plane and the (b) core microstructure consisting of primary (mainly mixed iron-chromium) carbides, nonmetallic inclusions in a tempered martensite matrix.
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Fig. 2: SEM micrograph showing the microstructure of the steel block consisting of alloy carbides dispersed in tempered martensite matrix. The presence of non metallic inclusions is evident.

Fig. 3a:

Fig. 3b:

Fig. 3: EDS spectra corresponding to characteristic types of inclusions found in the microstructure; (a) silicate and (b) sulfide (MnS).

 

Fig. 4a:

Fig. 4b:

Fig. 4: SEM micrographs at the area adjacent to fracture, presenting details of secondary crack propagation underneath the fracture surface; note the brittle transgranular cracking across the alloy carbide network. Decohesion processes occurred between the primary carbides and the matrix are also evident.

 

Fig. 5a:

Fig. 5b:

Fig. 5: SEM fractographs (SE imaging) highlighting the dominant fracture mechanism; (a) transgranular facets of alloy carbide islands alternated by dimpled areas, (b) a higher magnification detail (indicative boxed area) of the matrix area showing suppressed plasticity manifested by high dimple nucleation density (nano-sized dimples) surrounding secondary carbide particles.
Conclusion
The collected evidence suggests strongly that the failure of the clamping element was attributed to dynamic loading during service. However, detected flaws in the microstructure and material selection at high strength level depress toughness and constitute significant contributors to premature fracture. The potential revision of material selection and heat treatment condition together with the implementation of an appropriate Non Destructive Testing (NDT) technique for periodic inspection is suggested in order to minimize the risk of recurrence of such unexpected incidents which lead to serious machine interruptions.

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