case study
where the gradient switch on the “S” curve is then known as the DBTT with its correspondent energy absorbance. This means that virtually all metals would become brittle at some point during low external temperatures. Austenitic stainless steel bucks the curve There is an exception to this general rule. Austenitic stainless steel does not render a conventional curve on the DBTT graph, and the red line shows a typical austenitic curve. It is noticeable that the
condition to a brittle condition upon cooling through a critical range of temperatures.
• The corner of each hatch was square; these corners acted as points of stress concentration where cracks could form.
• To deliver the fastest possible construction speed demanded by the war effort it became necessary to revolutionise construction methods. This was accomplished by using prefabricated steel sheets that were assembled by welding rather than by the traditional time-consuming riveting. Cracks in welded structures may propagate unimpeded for large distances and when structures are riveted, a crack ceases to propagate once it reaches the edge of a steel sheet. • Weld defects and discontinuities where cracks could form were introduced by hastily trained and inexperienced welding operators. Stainless could have saved the day Could stainless steel have saved these vessels? The above lists of causes for the failure of these ships can be attributed to: • The material selection did not consider all the factors in the operating environment and the mechanical properties of the selected steels in this environment. • The design lacked basic engineering concepts relating to the avoidance of stress concentration at critical points in the structure of the ships. • There was a lack of technical skills and technological competency amongst the workforce to address production and manufacturing challenges. The lack of technical skills, suitable technology and processes, and design cannot be addressed by stainless steel, but it could have had an impact on the metallurgical reasons for the failures. The ductile-to-brittle transition process that led to the failures was alloy and temperature range specific. It is also an indication of a material’s level of toughness in certain conditions. Toughness is the ability of a material to absorb energy when subjected to an impact load. This is explained by the graph showing the Ductile to Brittle Transition Temperature (DBTT) for a specific material. At temperatures above (to the right of) the DBTT, the material is regarded as ductile, and the material will be tough. At these higher temperatures, the material will absorb elevated levels of energy during impact. As the material is cooled to temperatures lower (to the left) than the DBTT, the material loses its ability to absorb impact energy and becomes increasingly brittle and less tough. This “S”- curve is characteristic of most metals. The point
austenitic stainless steel grades do not exhibit the typical “S” curve, but only part of it. This part of the “S” curve never reaches a low temperature where the gradient of the curve will change and, as
such, does not have a DBTT. This means that austenitic stainless steel will remain ductile and tough even in cryogenic conditions at extremely low temperatures. The answer to the question would then be technically yes. Stainless steel could have saved the ships that failed due to brittle failure. Austenitic materials were developed in the early part of the 1900s and would have been available during the second world war. However, this is easy to state in hindsight. It should be remembered that at the time of the construction of the vessels, welding technology and material-making technology were not nearly as advanced as today. During the late 1930s and early 1940s stainless steel mills could produce the suitable grades for this application but did not have the technology to produce the required forms, i.e. the mills were not able to produce the large pieces of stainless steel materials with consistent metallurgical quality or flat products that could be used in the shipping industry. The North American stainless steel mills could also not produce the quantity of material required. As mentioned, welding technology standards for stainless steel were also very different compared to the modern methods we currently employ. Most stainless steel welding was still based on brazing and soldering. Welding quality would have disqualified stainless steel as a replacement material. We can conclude that stainless steel has been a solution to the world since its inception in 1913 for many a problem. Until recently we have never had a complete set of tools, skills, and knowledge to make full use of it. Who knows what the future holds for stainless steel, it is Simply Brilliant!
Issue 1 – 2023
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