The Influence of Aluminium and Silicon on Transformation Kinetics in Low Alloy Steels

Abstract

The use of aluminium to deoxidise steel is well known and has been the key to significant improvements in steel cleanness for some 100 years [1,2]. It is common practice to add more than sufficient aluminium to combine with the oxygen in the steel at the end of steel making to ensure current levels of cleanliness. Usually, therefore, steels made with current Basic Oxygen Steelmaking or electric arc practices contain excess free or soluble aluminium which can be exploited to an advantage by combining with nitrogen to form aluminium nitride, which then acts to refine grain size during subsequent processing. For most steels therefore, the aluminium additions made play several metallurgical roles; for example, the soluble aluminium to nitrogen ratio affects both the ferrite grain size and the strain ageing behaviour [3]. Many steel specifications therefore, pay particular attention to this aspect and Al/N ratio greater than 2 is normally considered to provide a fine ferrite grain size and freedom from the harmful effects of strain ageing on mechanical properties. As a direct consequence of these attributes, aluminium is usually in excess of the stoichiometric ratio of 2:1 and thus in many steels some aluminium is present in solid solution in the ferrite. Furthermore, the aluminium, invariably present in most modern steels is generally not considered to have an important or significant effect on microstructure. However, interest in the role of aluminium during microstructural evolution has been increased from work based on the formation of steel microstructures based on tough acicular ferrite in the weld heat affected zone (HAZ), thought to be nucleated by TiO compounds [4,5]. One approach which has been taken in the formation of such microstructures has been to adopt compositions which favour the formation of titanium rich inclusions, thought to be responsible for the nucleation of acicular ferrite [6]. Such steels are commonly aluminium free, or very nearly so, since a prerequisite for titanium oxide to form as inclusions is the presence of uncombined oxygen at the end of steel making. Observations made from studies into the microstructural development in weld metals appear to show aluminium having a deleterious effect on the formation of acicular ferrite, even in the presence of favourable Ti rich inclusions [7]. In weld metals, other alloying elements with marked nitride forming abilities, principally titanium, vanadium and boron, are present but there is a strong case for aluminium to be present in solid solution in ferrite. It is not obvious, as to what level the microstructure is affected, but the work by Thewlis indicates a marked change when the total aluminium content is greater than about 0.03 wt%, implying a reduction in the soluble aluminium content [8]. There have also been reports, that trace additions of aluminium less than 50ppm, can have a marked effect on the microstructure of laser welds [9]. Several other instances of a pronounced effect of aluminium on transformation behaviour have been documented, mainly in steels containing chromium and/or molybdenum additions [10-11]. No universal explanation has been offered for such effects. One interpretation suggests that partitioning of aluminium to the advancing ferrite/austenite interface reduces the boundary mobility leading to an increase in the hardenability of the steel [8,12].