stress checks on hardfaced screw element
12-01-2007, 09:28 AM,
#1
stress checks on hardfaced screw element
We have provided a coating by PTA powder welding on plastic extrusion screw elements Coating thickness is around 3-4mm diameter 300mm and width 50mm. coating materialis Ni-Cr-B-Si alloy by hoganas(1560 powder) having hardness of 58-59 Hrc. Base material is 34CrAlNi7V (C 0.3TO .37% Cr 1.5 to 1.8% Mo .15 to .2% Al .8 to 1.2% V .3%) Preheat and interpass temperature maintained at around 160 deg C. After grinding the coated surface we have observed lot of stress checks/cracks on the surface of less than 0.1mm thickness as tested by MPT/ floroscent DP. We conducted an ultrasonic Bonding test and observed no defects. The customer finds the job unacceptable due to presence of cracks as he is afraid of debonding of coating. Is there any literature indicating occurance of cracks in PTA available so that we can provide a support document for acceptance /or whether technically the job can Fail and hence should be rejected. I am looking forward to comments from the forum.
regards
Ragesh Bateriwala
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12-03-2007, 02:48 PM,
#2
RE: stress checks on hardfaced screw element
Hi Ragesh

I think you will find it hard to find a support document for acceptance. Are the cracks appearing before or after grinding?
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12-05-2007, 12:38 PM,
#3
RE: stress checks on hardfaced screw element
Gordon Wrote:Hi Ragesh

I think you will find it hard to find a support document for acceptance. Are the cracks appearing before or after grinding?
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12-05-2007, 12:42 PM,
#4
RE: stress checks on hardfaced screw element
rbateriwala Wrote:Dear sir,
Cracks are hardly visible and are apperaring during welding as indicated by sound. After grinding during a MPT (magnetic particle tset) we detected the presence of cracks.
ragesh
Gordon Wrote:Hi Ragesh

I think you will find it hard to find a support document for acceptance. Are the cracks appearing before or after grinding?
Reply
12-06-2007, 08:26 PM,
#5
RE: stress checks on hardfaced screw element
Hi Ragesh

Afraid I don't have first-hand experience with the PTA (Plasma Transferred Arc) process, though I suspect your substrate material may be have rather high hardenability. If you are getting a martensitic (hardening) reaction in the substrate on cooling, this will highly stress the coating.
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12-15-2007, 07:49 AM,
#6
RE: stress checks on hardfaced screw element
Hi Ragesh,
First of all, you have selected a self-fluxing alloy of high hardness meant for fusion by oxy-fuel torch at a low temperature where the base steel does not melt.The alloy is meant for brazing application rather than welding by PTA process.In welding due to melting of base materials,weld pool gets diluted by picking up of around 30% max elements from base metal, hence the composition of your first layer of deposit is completely different from that of the Hoganas alloy.However the extent of dilution in PTA can be as low as 5% by controlling welding parameters,which would enable you to get crack free deposit even in such high alloy, high hardness material.
You can get good idea on PTA process,dilution,applications and references from my book entitled 'Advanced Thermally Assisted Surface Engineering Processes' published by Kluwer Academic Publishers,NY (now taken over by Springer,NY, in 2004.For further details on the book you can go to the website on the title.
I was consultant to a party who is now regularly doing this job.
ram chattopadhyay
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