Weld-preheatingfor Crack-Safe Performance.SOLUTIONS with Effective, Practical AdviceWeld-preheating is a hot topic that should always be considered, even when it is finally decided that it is not needed in the specific application. By performing a search of the word "preheating" in this website only, one reaches more than 60 results, meaning that it is quite a popular argument, cropping up quite frequently. It is therefore proper to dedicate a separate page to this subject, to summarize its important points. Important AnnouncementWhen you need Welding-metallurgy knowledge (Sponsored Links) Preheat is used:
It is recognized that hydrogen induced cracking in welded steel structures is responsible for a large proportion of the failures reported, with a very large economic cost. The conditions resulting in hydrogen cracking are briefly described as those presenting "sufficient hydrogen and sufficient stress in a susceptible microstructure, at temperature below 150 0C". (*) Weld-preheating requirements, if prescribed by applicable Code or by engineering considerations, must be spelled out in Welding Procedure Specifications (WPS). In general, when welding mild steel, characterized by low carbon content and low hardenability, in applications without exceptional mechanical strength requirements or restrained joints, only a minimum Weld-preheating may be appropriate, depending on thickness. For less than 25 mm (1 in) preheat is generally not required except for drying if needed. For low alloy steels which develop higher mechanical properties upon heat treatment, a suitable range of Weld-preheating shall be specified, to avoid the rapid cooling that could dangerously affect joint microstructure and properties. The rate of cooling in the range from 800 0C to 500 0C has the highest influence in hardenable steels. For quenched and tempered high strength low alloy steels, it is imperative to establish the minimum but not to exceed the maximum temperature given by handbooks or by manufacturers. In fact a too high temperature Weld-preheating reduces the cooling rate too much, resulting in the transformation of austenite in ferrite (with islands of high carbon martensite) or coarse bainite, where both structures lack the strength and toughness required. Codes and Handbooks can give indications as to the recommended minimum preheat temperature to be applied for every thickness range, except that special conditions may require a thoughtful increase to improve results. A method involving the calculation of a factor called the Carbon Equivalent (CE) from the composition of a given steel (expressed as the weight percent results from chemical analysis) represent an attempt to reduce the influence of the amount of each of the elements present, to a single number. An accepted empirical expression of CE is that proposed by the International Institute of Welding (IIW) as follows: In the various methods using CE for determining the suggested Weld-preheating temperature, appropriate ranges of CE are established, with additional factors, to be taken into account, influencing the final selection. A summarized overview of the kind of information available from calculation of Carbon Equivalent for any given steel, and determination of Weld-preheating requirements is presented hereafter:
A complete treatment for establishing the required Weld-preheating temperature is available in the following reference: Furnace Weld-preheating is the best option if available, for iron castings and small assemblies. For larger structures other means must be provided, like banks of burners or heating mats of various shapes and types like electrical strip heaters, or induction or radiant heaters. The importance of knowing exactly the attained temperature before initiating welding cannot be emphasized too much. Taking reading from thermocouples or pyrometers, or even from temperature sensitive markers, should be possibly performed from the opposite side to that of heat application. The temperature distribution should be as uniform as possible for a vast zone around the joint place. Selecting suitable Weld-preheating temperature will provide sound weldments without cracks and will contribute to successful economic performance of any welding operation. See also: Any questions or comments or feedback? Write them down and send them to us by e-mail. Click on the Contact Us button in the NavBar at top left of every page.
To reach a Guide to the collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics. Case Hardening
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