On site Machining - Welding, Stress Relief and NDT Case Study Steel Tube Mill Hydrotester

Overview

The 15,000 PSI Hydrostatic Tube and Pipe testing mill sealing surface heads, at a major manufacture of pipe and tube, had become cracked. The surface of the fill and purge head blocks in the hydrotester require a fine, indication-free finish on the sealing surfaces. The surfaces had become cracked to such an extent that, while the testing machine could still maintain a 15,000 PSI hydrostatic test pressure, excessive amounts of water were being wasted during the test process. The material of the head blocks, AISI 4340 Nickel - Chrome - Moly, requires specialized weld media and pre and post weld heat treatment procedures in order to prevent cracking. During the preliminary client interview Furmanite learned that the prior vendor's repair was completed using local torch preheat only, standard carbon steel compatible electrodes and without post weld stress relief. Furmanite advised the client to be prepared for indications of discontinuity deeper than expected due to the uncontrolled processes previously used which we expected to be revealed after the initial pre-machining process.

 

Solution & Implementation

 

PT check during Pre-Machining

Post Weld Machining

Heat Treatment

Furmanite offered an engineered six step controlled process as a solution, which was reviewed and accepted by the owner.

1. Step one:  An On-Site machining clean up cut utilizing a Furmanite manufactured Silk^® portable facing machine of the 22" diameter sealing surface of the heads commenced, with periodic PT until it was verified that all indications of discontinuity (cracks) were removed. The pre-machining excavation depth was roughly twice client expectations, due to the errant weld methods used previously-certain indications were deep enough to require hand grinding, to preserve as much parent metal as possible.
2. Step two:  The fill and purge head blocks were preheated and the temperature held.
3. Step three:  The critical 22" diameter sealing surfaces of the heads were welded with the proper low hydrogen electrodes, per weld procedure requirements. Sufficient material was applied raising the sealing surface higher than the OEM finish surface height.
4. Step four:  The facing machine was repositioned to the datum points and the overlay of weld was machined to a flatness tolerance 0.001" overall.
5. Step five:  The sealing surfaces were honed to create a surface finish of 32 RMS, which was required to satisfy the sealing criteria.
6. Step six:  The fill and purge head blocks were post weld stress relieved and then ambient air quenched.