Examination of cracked weld samples enabled TWI's Materials Department (now the Metallurgy, Corrosion and Surfacing group) to diagnose a problem in a heat exchanger and has led to recommendations for improved design and manufacture.
During a routine hydrostatic pressure test, Industrial Member, Wykes Engineering, found cracked welds in a customer's austenitic stainless steel plate heat exchangers. The heat exchanger consisted of a series of large hollow section rotating discs which, during service, were filled with steam injected at a temperature of 164◦C and pressure of six bar. A subsequent ultrasonic inspection revealed fractures at ten reinforcing bolt welds situated in the mid-third of the horizontal disc assembly. In an attempt to identify the cause of the failures, fractured bolt samples were removed from one disc and sent to TWI for investigation.
Visual examination of the fracture surfaces revealed characteristic fatigue cracking beach marks. Several discrete groups of beach marks were seen indicating that cracking had initiated at a number of locations on each bolt. Regions of the original machined joint preparation were seen on the fractured components, suggesting that weld root penetration was variable.
A through-thickness section, removed from an unbroken weld, revealed that cracking had initiated close to the weld root both in weld metal and in parent material. The defects consisted of large transgranular cracks typical of fatigue cracking and smaller subsidiary branched cracking showing features consistent with stress corrosion cracking.
Examination of the fracture surfaces in a scanning electron microscope revealed fine striations indicative of fatigue crack growth. Corrosion deposits on the fracture surfaces were also analysed qualitatively in the SEM, using an energy dispersive X-ray attachment. The major alloying elements of the austenitic stainless steel were identified, along with traces of calcium, potassium and chlorine.
Based on this examination, the conclusions were:
- Failure occurred by fatigue with contribution from chloride stress corrosion cracking.
- The welds were of acceptable quality, but the fatigue strength was inadequate for the service conditions.
TWI recommended that TIG welding would improve the weld root profile leading to increased fatigue strength. It was also proposed that, by modifying the bolt design to move the weld away from an area of highest stress concentration,an acceptable weld root profile could be produced. This design change would improve the fatigue and corrosion resistance of the joint.