Opportunities and dangers of using residual elements ... - Jernkontoret

5.2.2 HardenabilityReference [38] gives a summary of the most important variables, which influencehardenability, which are austenite grain size and composition, the second being discussedin this chapter.Carbon has a marked influence on hardenability, but its use at higher levels is limited,because of the lack of toughness which results, the greater difficulties in fabrication and,most important, increased probability of distortion and cracking during heat treatment andwelding.Likewise, most metallic alloying elements slow down the ferrite and pearlite reactions, andso also increase hardenability. Exceptions are Co, because it increases the rate ofnucleation and growth of pearlite, Ti because it reacts with C to form TiC and S because ofMnS.The most economical way of increasing the hardenability of plain carbon steel is toincrease the manganese content, from 0.60 wt% to 1.40 wt%, giving a substantialimprovement in hardenability. Chromium and molybdenum are also very effective, andamongst the cheaper alloying additions per unit of increased hardenabilily. Boron has aparticularly large effect when it’s added to fully deoxidized low carbon steel, even inconcentrations of the order of 0.001%, and would be more widely used if its distribution insteel could be more easily controlled.The effect of the main residual elements (Cu, Cr, Ni, Mo, P, Si, Sn) on the hardenability oflow alloyed C-steels (XC10, XC42, 20M5) is also given in reference [39]. The effect onhardenability was ranked as follows: P>Mo>Cr>Si>Ni/Cu. The effect of Ni and Cu isneglectible under 0.3%. From that, it was assessed that elements coming from scrap (Cr,Si, Mo) allow enonomy of adding elements such as Cr, often necessary to increasehardenability of carbon steel produced from ore.According to reference [28], copper somewhat increases the allotropic and martensitetransformation temperatures, the increase of temperature being proportional to its content.As an element weakly stabilizing austenite, copper impedes the decomposition of austeniteand thereby increases the hardenablility of steel.In reference [40], the derivation of carbon equivalents to assess hardenability of steel hasbeen examined. This equivalent was derived from the incubation time before thetransformation to ferrite, pearlite or bainite, which is illustrated in Figure 22 and is given inthe equation below (note that the 1,8 is written 1·8 and so on):It was compared with experimental results from other studies, which is shown in Table 7:The comparison reveals that the calculated coefficients for silicon, manganese, copper andnickel in the carbon equivalent equation are in good agreement with the experiments,whereas the agreement for the molybdenum and chromium coefficients is poor.35