Thermal Fatigue and carbon steel microstructure

[SMF1964]

Anyone have thoughts on the fatigue behaviour of spheroidized steel would be different than that of a clean ferrite & pearlite microstructure? The material in question is a carbon steel boiler tube that is subject to thermal fatigue cracking. The structure is fully spheroidized (during fabrication, not in-service) and the question was raised as to whether this would make it more susceptible to the fatigue cracking.

My thinking is that any effect would be minimal in practical terms, but would like to know if anyone has a different opinion/actual experience related to this.

SMF, out

 

[metengr]

SMF;
My belief is that the thermal fatigue resistance for the spheroidized carbon steel boiler tube would be slightly reduced in proportion to the reduction in tensile properties (UTS, YS) versus a ferrite/pearlite microstructure in a carbon steel tube.

I found this interesting parameter that has been used to evaluate thermal fatigue resistance of metals;

TFP = yield strength x thermal conductivity/Young's Modulus x linear coefficient of expansion

 

[SMF1964]

Metengr: Do you have a reference for the TFP? I'd like to reference it in the technical report and a published to to-be-published reference holds a bit more weight than "I saw it on the internet", if you know what I mean. ;-)

The hardness of the base metal in this area is only RB 72, which puts the tensile strength extrapolated at 55-60 or so (it's off the scale below, but a curve fit through the data points to that level).

 

[TVP]

SMF1964,

ISO 18265, which was derived from DIN 50150, has an extended range of hardness and tensile strength conversions, with the low end of the scale being 255 MPa = 80 HV or 78 HB. 72 HRB is ~ 420 MPa or ~ 60 ksi.

 

[metengr]

SMF;
Can't you simply reference "the internet"? Ok, here is the reference

"Application of Molybdenum Metal and its Alloys" report for the International Molybdenum Association of America (IMOA) prepared by Dr John Shields, Climax Specialty Metals, Cleveland, Ohio (1995). The thermal fatigue resistance parameter is described and referenced on page 6.