Tom First : Doing repair welding on used Stellie 6 layer, either worn out or with cracks, would give problems of repetitive cracking . Never attemp this. For repair welding on used butterfly or gate valve discs - low alloy steel variety of either AISI 4140 or ASTM A 182 F 22 V - you have to be careful from the following considerations ; A 182 F 22 or 4140 variety is air hardening grades with low hardenability and low impact strength. It will be still worse if it is with F 22 V. When repair welding on used air hardening low alloy grade materials (Cr-Mo or Cr-Mo-V grades), you have to remove cracks totally, NDT inspect the edge prepared disc fully by WFMPT, then only attempt repair welding For successful repair welding, especially when Stellite 6 application is involved, you have no other go - other than pre-heat to 250-280 deg cent (4550-500 o F) first before welding. PWHT is also required to reduce tensile stresses as much as possible for a hard facing Stellite 6 application. Never attempt without PWHT on a welded low alloy steel with Stellite 6 application. Usually, on a cracked or worn out valve disc with low alloy steel material grades, it is preferable to give a buttressing layer of Inconel 625 or equivalent. A buttressed non-ferritic grade (austenitic base alloy with buttressed Inconel layer) will give better successful repair welding using Stellite 6 or Stellite 21on low alloy vavle discs / tems. A buttressed layer - to be on the safeside - is given usual pre-heat to 250 deg Cent and then also PWHT for additional application of Stellite 6 alloy by GTAW process. This is a better and more fool proof approach even for buttressed Inconel 625 layer welding done before application of Stellite 6 or Stellite 21. Yes, theoretically, with buttressed Inconel 625 welding layer on Low Alloy steel (AISI 4140 or ASTM A 182 F 22 or F 22 V), you may not need PWHT. Still from successful and long term use of Stellite 6 layer PWHT helps even a buttressed weld layer of Inconel 625. over Alloy F 182 F 22 . It is tedious, no doubt, for a maintenance person and for continuous repairs on valve discs by grinding to remove all cracks in F 22 disc, pre heat, give a buffer layer of Inconel 625, carry out PWHT before application of Stellite 6 hard facing over lay. In a way to simplify, the PWHT on buttressed Inconel 625 can be by-passed after a thorough NDT Inspection by DP check and then you could try either Stellite 6 or Stellite 21 hard facign. Stellite 21 is a little softer hard facing alloy compared to Stellite 6 and may stand the same duties (same steam service - without erosion - corrosion of disc or failure of stems) and comparatively easier than Stellite 6. If your valve orifice is narrow and the valve is throttled in service quite often, then Stellite 6 would give a better long term option than Stellite 21 repaired discs. It is a question of maintenance philosophy - whether you are initially prepared to take a better and more long term application in mind with successful repair on used low alloy steel disc or stem ( from long term maintenance aspect) or take a short cut from simplifying welding procedures and heat treatment conditions to get less longivity in repaired discs by hard facing (especially in using Cobalt based alloys). Probably, you can get away with PWHT with a buttressed Inconel 625 and with good inspection of the buttressed disc and then do with proper pre-heat and post heat needed for a good and successful Stellite 6 alloy or Stellite 21 - whichever option you may decide from maintenance angle. Trust this helps you for successful repair of used low alloy steel discs in steam service . Best of luck in your attempts. C.V.Srinivasan Nishi Engineers Pvt Ltd India March 24, 2007 E-mail: nishi@vsnl.com >Presently our steam service valves (butterfly, gate and plug >type valves) have a %7E.040" thk layer of Stellite 6 layer >GTAW welded to a chrome alloy base (typically 4130, but we >also use an ASTM A182, grade F22V forging ). I have seen >it suggested in a different media that when we overhaul or >repair our valves, that we should first apply an Inconel >layer (625 was recommended), then PWHT, then apply our >Stellite 6 overlay. The claim was that the combination >would reduce crack susceptibility. > >I am asking for other opinions on the above recipe, in >particular hoping for metallurgical explanations, bulletins >or other references on why this is an effective combination. > >My reason is that our pure Stellite 6 application is very >tricky and time consuming for us so we seek to develop an >alternate process that will provide more reliable result in >production and inservice. > >Thankyou very much for allowing me to post my question. >tom

Nowadays thermal spray process is become importance in maintenance application for repair/alteration. Even Titanium or tungsten carbide can be overlaid using thermal spray method without much difficulty and without PWHT. Many indian vendors are carrying out Thermal spray application successfully. regards,