23 Apr Weld parameters for a high purity stainless steel gas system: What is “acceptable”?
Judging welds for acceptability is somewhat subjective. There are some generally agreed industry standards such as SEMI F81 for Visual Inspection and SEMI F78 for Gas Tungsten Arc Welding but these are not based on scientific studies correlating weld quality to gas purity. For the most part they represent what is achievable by a welding technician practicing the state of the art. The most subjective parameter is oxidation at the heat effected zone.
The parameters that are judged are: penetration, bead width, bead depth and convexity or concavity at the inner wall, bead “meander”, porosity/inclusions/slag, weld bead overlap, consumption of tack welds if used, amount of discoloration (oxidation) of OD and ID in heat effected zone, misalignment (axial and circumferential), and down-slope termination. For each of these parameters, SEMI F81 provides guidelines. For example, for inner weld bead width, SEMI F81 states that the minimum width shall be between 1.0 and 2.5 times the wall thickness and the maximum width shall be no more than 1.25 times the minimum width.
Some owners and some Quality Assurance Representatives use their own guidelines – either more or less stringent than SEMI F81. To avoid problems during execution of a project, the Contractor should create a set of weld samples prior to project award that are agreed as “acceptable” by all parties (Client, Contractor, Quality Assurance Representative).
For a system to be cost effective, the material specification, installation method, welding acceptance criteria, and final testing acceptance criteria need to be balanced. Experience is required to get this right.
Ralph Cohen, P.E., who provided this information, is an engineering consultant to customers for gas systems components and HVAC, specializing in semi-conductor and other industrial manufacturing facilities. He has over 30 years in the field, and continues problem solving for clients through his consultancy. More information on Ralph’s project work, publications, etc. can be found here RalphCohenConsultancy or call 971-227-8989.