Bambie
Electrical
- Mar 31, 2012
- 242
With reference to thread124-450268, LP examination of the 17-7PH or S301 knife blade ends has revealed unusual mind-plane cracking (see pictures on word document attached).
Is it possible that surface hardening heat treatment created layers having different Modulus of Elasticity (E) and thermal coefficient of expansion (Z)?
It seems reasonable that the blade sides and ends would have higher E and lower Z than the core or mid-plane layer.
When the blade cools, the mid-plane layer should contract more than the side layers and be left in a state of tension, especially along the longest dimension – the blade axis.
The blade sides were then fillet welded to the plain carbon steel blade holder.
In-service superheated steam would cause the holder to expand further than the cooler blades, increasing tension in the blades.
Thermal cycling would eventually initiate cracks in the blade welded ends at the mid-plane to outer layer interface.
Does this simplistic model explain the mid-plane crack orientation?
Would the fillet welds have modified the blade metallurgical properties in a way that would produce the unusual mid-plane crack orientation?
Is it possible that surface hardening heat treatment created layers having different Modulus of Elasticity (E) and thermal coefficient of expansion (Z)?
It seems reasonable that the blade sides and ends would have higher E and lower Z than the core or mid-plane layer.
When the blade cools, the mid-plane layer should contract more than the side layers and be left in a state of tension, especially along the longest dimension – the blade axis.
The blade sides were then fillet welded to the plain carbon steel blade holder.
In-service superheated steam would cause the holder to expand further than the cooler blades, increasing tension in the blades.
Thermal cycling would eventually initiate cracks in the blade welded ends at the mid-plane to outer layer interface.
Does this simplistic model explain the mid-plane crack orientation?
Would the fillet welds have modified the blade metallurgical properties in a way that would produce the unusual mid-plane crack orientation?