Failure analysis of tower axle in the wet drawing machine

784
Advanced Materials and Processes III

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The cutting samples are respectively put into two groups. One group is conducted tempering heat 
treatment; the other is without heat treatment. As shown in Table 1, 40Cr micromorphology is 
observed by metallographic. 
Table 1 Micromorphology of 40Cr 
location
the edge
the subcenter
the center
before quenched and 
tempered treatment 
after quenched and 
tempered treatment 
From Table 1, it can be seen that organizational change trend from the edge to the center of 40Cr 
axle is shown before the heat treatment. Based on the size of the organization, in the process of 
cooling the organization have obvious difference as a result of different cooling speed from edge to 
center. Due to the cooling speed faster, organizations of edge area are much smaller than the center. 
It can be seen that ferrites appear more after quenched and tempered treatment. The volume 
fractions of ferrite are calculated by using image tool graphical tool. The results are 7.52%, 7.52% 
and 29.49%. Thus, it can be seen that volume fraction of ferrite shows a trend of increase from edge 
to the center in the axle. 
According to data shows [8], reticular ferrite should not have are present in the sample after 
quenched and tempered treatment. According to the requirements, surface microstructure of the 
carbon structural steel and alloy structural steel parts are commonly tempering sorbite and less than 
or equal to 5% of the ferrite after quenched and tempered treatment. Studies have shown that [9], no 
matter which form the ferrites exist in, they will reduce the using performance of steel. Especially 
the ferrite distributes along the grain boundary, it not only makes the intensity drops, but also 
affects the fatigue fracture problems directly. 
Hardness analysis 
Variations in hardness from edge to center of 40Cr axle are measured by Brinell hardness tester 
before and after quenched and tempered treatment. The hardness changes are shown in Fig. 6. 
As shown in Fig. 6, the black square dotted line indicates hardness change trend of 40Cr axle 
before the quenched and tempering treatment. It can be seen that hardness change trend from edge 
to center of 40Cr axle basically follows the quadratic curve equation. Edge hardness is 225 HB and 
that is the maximum. The hardness becomes the smaller when it gets closer to the center. At last, 
hardness of the center is 182 HB. The main reason of different harnesses between edge and center 
of 40Cr axle is the uneven inter-cooled speed in cooling process. In result, the organization of edge 
is relatively uniform fine, and the hardness of edge slants big. By contrast, the organization of 
subcenter is relatively bulky, and hardness of subcenter is low [10]. Due to the nonuniformity of 
hardness distribution and the average hardness is short of the required 220 HB, proper heat 
treatment process is required to eliminate the nonuniformity of structure and hardness.