PSI - Issue 5

Lars Sieber et al. / Procedia Structural Integrity 5 (2017) 1011–1018 Sieber et al. / Structural Integrity Procedia 00 (2017) 000 – 000

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1. Introduction

Although many structures made of old mild steel built in the period between 1890 and 1940 are still under load after decades of service, there is usually no need to replace them. When assessing these existing steel structures to be able to decide on necessary rehabilitation and reinforcement measures engineers requires information about the mechanical as well as the technological properties of the material. During the rehabilitation and strengthening of existing steel structures, almost exclusively connections with close tolerance bolts are currently used. In addition to the considerable effort for the preparation of these fitted connections, the additional holes lead to a further weakening of the cross sections which actually have to be reinforced, above all in the case of tensile-stressed components. Whereas puddle steel is generally classified as non-weldable, the weldability of old mild steels is limited and only feasible under specific conditions. A suitable welding procedure for use on construction sites is the manual metal arc welding with basic-coated stick electrodes. The filler material reduces the tramp elements and provides ductile welds. In order to minimize the risk of brittle fracture, Kühn et al. (2008) recommended avoiding the use of welded connections in old steel structures if possible. If for constructional reasons welding is preferred (e.g. Stroetmann et al. (2009) or Möll (1997)), the metallurgical characteristics of the steels have to be considered. While the inner high impurity content is generally considered a show stopper for welding, only few practical welding result surveys are known that rely on actual parts of old structures using usual connection types. While the investigations by Lüddecke (2006) are restricted to weld seams on the component surface using the rim layer, it was verified by Möll (1997) and Möll (2000) that also butt welding of old mild steels is possible despite the melting of the segregation. However, the weldability of the components had to be verified by means of extensive material investigations of all components to be welded, by means of process tests and welding tests on the structure. Based on the above mentioned literature, the following scenarios are feasible as an answer whether such olds steels are generally weldable or not: A) Impurities generally prevent acceptable welding, only (unrealistically) clean compositions allow welding. B) Acceptable welding results can be reached on steels with special filler materials and/or within certain chemical limits which are typically (or at least often) encountered in such old steels. C) Most steels are weldable with acceptable results with standard filler material, only rare “dirty” steels show defects. 2. State of the art It is already known from the investigations by Kühn et al. (2008) that the chemical, metallurgical and mechanical characteristics of old structural steels have significant variations. In order to cover the widest possible range of steels, material samples from different structures, components and construction years were used for the investigations. Six material samples of mild steel were selected from almost 30 available components (years of fabrication: 1888 - 1930). The material was primarily selected by its chemical composition (see Table 1). Material with high concentrations of sulfur, phosphorus and nitrogen were selected since these tramp elements have a significant influence on the weldability. With the introduction of the welding technology in steel construction in Germany in the 1930s, a further development of the material standards for structural steels was made in order to ensure the weldability of the steels. Beside a sufficient toughness, the content of individual tramp elements was limited. Table 2 shows the maximum contents of these elements from three representative material standards. Comparing the limits with the results of Table 1, it is obvious that three of the six mild steels were declared as weldable. 3. Experimental research for determining the weldability 3.1. Selection of the sample material