Laser welding and subcriticai annealing of a 2Cr-1Mo steel for pipes operating at high temperatures

Abstract

Experiments were performed to perfect a process for the CO ₂ laser welding of an ASTMA335 grade P22 steel for pipes operating at high temperatures. The corresponding conventional welding techniques require a rather complicated schedule that includes both pre- and post- heating to prevent cracks and martensite formations in melted and heat affected areas, and has a mean duration of several hours. Qualification of the laser-welded joints was performed according to section IX of the ASME standards. The following results were obtained:

-      defect-free joints without any form of preheating;

-      a post-weld subcriticai annealing process able to reduce the hardness of the melted area to 280 HVin a few minutes (the hardness of the as- welded melted zone is around 400 HV); a similar process was applied at the end of the weld pass by modulating the power density delivered to the welded joint.

Correct temperature distribution during annealing was ensured by means of a control system using a two-colour optical pyrometer focussed on the joint surface, and computerised checking of the incident laser power.