Control of weldability

The understanding of the hardenability is important in steel development with respect to weldability of steels as well as to the design of quenched and tempered steels. The common way to judge if steel is suitability to welding is the use of a carbon equivalent, which reflects the alloy content to the hardenability of the heat affected zone (HAZ). Most common of these equivalents is the IIW carbon equivalent which has been in use for decades. However, this is an empirical equivalent, developed for CMn steels, and it doesn’t say anything about the mechanical properties obtained in the HAZ. As a guideline a maximum HAZ hardness of 350 HV is normally put to avoid cold cracking in the HAZ. Numerous carbon equivalents have been published during the years, but few have been accepted and used. A drawback of all these equivalents is that they have been evaluated for a set of chemical compositions which may be quite narrow, or that they have been evaluated on steels produced having other cleanliness, residual element levels etc. as is common in modern steelmaking. The hardenability of steel designated to quenching and tempering is normally evaluated using either Jominy testing or using a calculation using the Grossmann formalism. As the hardenability described in the carbon equivalents used in welding and the hardenability of quenched and tempered steels using the Grossmann formalism are two different descriptions of the same phenomenon the aim of this thesis is to link these two formalisms together and use the knowledge from the HAZ hardenability to design quenched and tempered tool steels. The goal is to produce such steel grades having a much better weldability as compared with standard grades available at the market. In this work the hardenability in the heat affected zone (HAZ) in weldments in high strength low alloyed steels (HSLA steels) having a yield strength of 350 MPa. This part of the study includes microstructural studies of the HAZ and the influence of microalloying elements on the phase transformations which take place during cooling from the peak temperature of the welding cycle. These results have been linked to the mechanical properties of the HAZs. In the second part of this thesis the development of tool steel grades with respect to optimize both hardenability with respect to; matrix hardenability (i.e the Grossmann approach) an enhanced weldability of such new steels due to a lower alloying content than is usual in such grades. These new steels also possess a superior Machinability as compared ton the common standard grades W.nr 1.2311 and W.nr 1.2312.