Metal Arc Welding (GMAW, also known as MIG welding) and Tungsten Arc Welding (TIG welding) are two common methods of joining thin wall sections. Welding can change material properties (strength, ductility, and hardness) in the heat-affected zone (HAZ).
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When the yield strength of pipes is less than 460 N/mm2, the influence of welding on the pipe properties in the HAZ does not seem to be significant (Zhao and Jiao 2004). All welding processes must be pre-qualified according to certain standards, e.g. B.AS/NZS 1554.1 (Australia Standard 2000) in Australia. AS 4100 defines two categories of construction welds, namely SP welds (for structural purposes) and GP welds (for general purposes). When testing macro sections according to AS 2205.5, more stringent requirements are required for SP welds.1 (Australian Standards 2003a). A higher coefficient of performance is specified in AS 4100 for the SP weldment than for the GP weldment.
Earlier versions of Australian Steel Construction Standard AS 4100 (Standards Australia 1990) only applied to steel 3mm thick or thicker. Pipe sections less than 3mm thick could not be designed to this standard and had to be designed to AS 1538-1988 for cold formed steel structures (Standards Australia 1988). As described in Chapters 3, 4, 5 and 6, element design rules have been developed and incorporated into AS 4100 (Standards Australia 1998) for pipe cross-sections less than
mm, as summarized by Zhao and Hancock (1998).However, the design rules for welded joints in AS 1538-1988 may not be appropriate for elements less than 3mm thick. The design principles of AS 1538-1988 were the same as the American Iron and Steel Institute (1986) specification, which was based on tests performed on a steel plate by Peköz and McGuire (1981). Similar studies on welded steel plates were also carried out by Stark and Soetens (1980), which were adopted in Eurocode 3 Part 1.3 (2004). Because they are derived from
flat steel plates, these design principles may not be appropriate when applied to RHS.The thickness limits for the design of welds in the different standards are between 3 mm and 4.6 mm, as summarized in Table 7.1. In general, there are two types of standards. One is for sections above the thickness limit and the other for sections below the thickness limit.
|America||AISC specification (1999)||≥ 4.6 mm (or 3/16 inch)|
|NAS (2004)||< 4.6 mm (or 3/16 inch)|
|AWS D1.1 (2002)||≥ 3.2 mm (or 1/8 inch)|
|AWS D1.3 (1998)||< 3.2 mm (or 1/8 inch)|
|Australia||AS 4100-1990||≥ 3 mm|
|AS 4100-1998||≥ 3 mm except cold-formed tubes for which all thicknesses apply|
|AS 1538-1988 AS/NZS 4600-1996||< 3 mm < 3 mm|
|Canada||CSA-S16-01 (2001)||≥ 3.5 mm|
|CSA-S136-01 (2001)||< 3.5 mm|
|Europe||Eurocode 3 Part 1.1 (2003)||≥ 4.0 mm|
|Eurocode 3 Part 1.3 (2004)||< 4.0 mm|
Investigations on welds in cold-formed thin-walled RHS were carried out by Zhao and Hancock (1995a, 1995b, 1996) and Zhao et al. carried out. (1999). The aim was to integrate the design rules for welded joints smaller than 3 mm into AS 4100-1998 (Zhao et al. 1996). Three types of welds were tested, namely butt welds, transverse filet welds and longitudinal filet welds as shown in Figure 7.1. The nominal RHS yield strengths used in the test program were 350 N/mm2 and 450 N/mm2.All welding procedures are prequalified to Section 4 of Australian Standard AS 1554.1-1991. For butt welds, only those with full (or full) penetration were tested. For filet welds, it has been found that the actual weld size (arm length) is approximately 1.5 times the nominal weld size.This corresponds to the trend identified by Pham and Bennetts (1983) after analyzing data from an international test set involving 10 countries (Ligtenberg 1968), as shown in Figure 7.2. The oversized weld was used in filet weld design template calibration. The weldment rules for thin sections were calibrated using the reliability analysis technique used to calibrate AS 4100-1998 for thicker sections. As explained later in this chapter, the design principle of BS 5950 Part 1
is similar to that of AS 4100 for the design of butt welds with full (or full) penetration, i.e. H.Volume. Checking the resistance of the base metal. The design principles of BS 5950 Part 1 are also similar to those of AS 4100 for the design of filet welds, i.e. H. checking the strength of the weld metal.