WELDING WEAR PLATES

Here is a recent application that you might find interesting. The user was trying to weld 50 mm thick Hardox® 400 pads to a mild steel plate. He had switched from the SMAW (Stick) process to GMAW (MIG/MAG) process. He was welding with hardfacing wire and the weld kept cracking, so he asked how to prevent this from happening.

Hardox® 400 is an abrasion-resistant steel with a hardness of approximately 400 Brinell. This steel is typically used on machines and implements that are exposed to abrasive wear. Often, wear plate will be welded to a plain carbon steel structure only in the areas that experience abrasive wear. Since wear does occur, wear plates need to be replaced frequently, which makes this a very common welding question.

Although these materials are very weldable with many processes, there are three things to consider when welding wear plate to plain carbon steel: (1) cracking in the weld; (2) cracking in the heat affected zone (HAZ); and (3) softening of the base material around the weld.

In the question here, the user wanted to weld with hardfacing wire to join the wear pad to plain carbon steel, and to obtain abrasion resistance at the same time. The hardfacing filler metal selected by this user is too brittle and is susceptible to cracking especially on this thick base metal. To avoid weld cracking, this highly restrained joint needs a welding procedure that uses preheat, filler metal with lower strength than the base metal for the root and filler passes (like a 307 stainless steel), and then hardface only on the cover pass to obtain abrasion resistance of the weld surface.

Cracking in the HAZ can be avoided with the use of preheat and a low-hydrogen welding process like GMAW (MIG/MAG). The user should consult the wear plate manufacturer for proper preheat for the thickness of the wear plate to be welded. Typically lower preheat is recommended to avoid softening and degrading the wear resistance of the base material.

Softening of the base material around the weld is normal, as the base metal near the weld reaches critical tempering temperatures. The goal in welding is to reduce that size of the soft zone around the weld by using the lowest possible preheat, and controlling the welding process to maintain the lowest possible heat input and a maximum interpass temperature. For the application above, that means: (a) preheating to approximately 75 deg C (167 deg F); (b) keeping heat input under 2.5 kJ/mm (64 kJ/in) by making multiple, smaller weld beads; and (c) maintaining the base metal interpass temperature below 175 deg C (350 deg F).

In this application the GMAW (MIG/MAG) process is a great idea to improve productivity, but the filler metal selection must also be correct to avoid weld cracking.

On another project, AWS offers prequalified joints and WPSs in D1.1, Structural Welding Code ? Steel that can save users time and the cost of qualifying welding procedures. One user who performs non-structural welding work asked if there was something like this available for him.

Yes, there is. AWS offers over 60 Standard Welding Procedure Specifications (SWPSs). Each SWPS is supported by multiple procedure qualification records (PQRs) complied and reviewed by the Welding Research Council. SWPSs are available for plate, pipe, and sheet metal applications for many filler metal types, material thicknesses, welding processes (including SMAW, GTAW, GMAW, FCAW), and base metal types (including carbon steel, CrMo steels, and stainless steels).

AWS currently sells SWPSs for $186 to AWS members and $248 to non-members. Reference pages 20-21 of the AWS 2009 Publications Catalog for the complete listing of SWPSs. Many codes and standards have adopted SWPSs, with certain restrictions, and may require the engineer's approval:
? AWS B2.1, Specification for Welding Procedure and Performance Qualification
? AWS D1.1, Structural Welding Code – Steel
? AWS D1.2, Structural Welding Code – Aluminum
? AWS D1.3, Structural Welding Code – Sheet Steel
? AWS D1.6, Structural Welding Code – Stainless Steel
? AWS D9.1, Sheet Metal Welding Code
? AWS D15.1, Railroad Welding Specification – Cars and Locomotives
? ASME Boiler & Pressure Vessel Code – Section IX. A demonstration test weld may be required. For more details on using SWPSs for ASME work, reference these additional SWPSs and these SWPSs.

Using a SWPS, savings may be realized by eliminating the need to weld a test plate/pipe and perform destructive testing to meet procedure qualification requirements. However, testing still may be required to qualify welders. SWPS does not replace or substitute for fabrication codes, specifications, contract requirements, or capability and judgment on the part of the user. A WPS is to be used only as permitted by the applicable fabrication code, specification, or contract document.

If you pay attention to details, SWPSs can be a great tool to save the time and cost of qualifying welding procedures.

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Kevin Beardsley, a registered professional engineer with 21 years of manufacturing experience, is an application engineer at Lincoln Electric , 22801 St. Clair Avenue, Cleveland, OH 44117-1199, www.lincolnelectric.com. For questions or comments on this column, contact Kevin at 216-383-2259 or [email protected].