Peter Adcock Cr-Mo Shell and Alloy 600 Tube Exchanger You have not mentioned the process conditions - Fluid or Gas on the shell side and tube side, Design temp and Design pressure. Cr-Mo materials require PWHT at 700 deg cent + 10 deg cent (depending on the grade of Cr-Mo used for this exchanger). It is P5 or P3 grade then PWHT will be around 720 + or - 10 deg cent. 600deg centigrade is too low for Cr-Mo materials unless you wish to low temp stress relieving from fabrication angles due to higher thickness or from non-availability of site PWHT machines. Alloy 600 tube in such exchangers should be strength welded instead of double groove. Stength welded tube to tube sheet may require PWHT if the tube sheet material is Cr-Mo to avoid high hardness after strength welding in the HAZ of tube to tube sheet weld. Usually the Cr-Mo tubesheet is given Alloy 600 weld over-lay to over come this problem and then the whole shell side is PWHT in a furnace before tube is inserted. Welding Alloy 600 with Alloy over-lay welded 600 material on Cr-Mo tube sheet may not require PWHT for strength welding done between Alloy 600 tube and Alloy 600 tube sheet over-aly weld Trust this answers your question C.V.Srinivasan Nishi Engineers Pvt Ltd Chennai 20 India August 12,2004 E-mail:nishi@vsnl.com >We are manufacturing a Shell and tube Heat Exchanger, Cr Mo >on the Shell side and alloy 600 on the Tube side. The Cr Mo >requires Post Weld Heat Treatment at around 600 deg c. Will >this temperature affect the corrosion resistance of the >alloy 600 tubes? If the tubes are left out for the PWHT >distortion may prevent their insertion afterwards. We could >change the Shell side material to St Stl to obviate the >problem but this would be a costly exercise.