What are low alloy steels, and what precautions should I take when welding them?Frequently Asked QuestionsLow alloy steels contain a few percent (typically between 1 and 7%) of elements such as Cr, Ni, Mo and V. This category includes chromium steels (containing up to 5% Cr and 1% Mo) and nickel steels (containing up to 5% Ni).Low alloy steels are generally weldable (seeWhat is weldability?), but it is important to know the service, joint configuration and the subgroup of the material type. Low alloy steels can be welded by most processes, as long as adequate precautions are taken to avoid defects. It is important to know the composition of the material, either from a mill sheet or a dedicated chemical analysis, as composition influences weldability significantly.With increasing carbon or alloy content, low alloy steels generally become more difficult to weld as the heat affected zone hardness increases. The need forpostweld heat treatment(PWHT) of these joints also increases. The composition is also important in identifying high, but allowable, levels of residual elements such as sulphur or phosphorus, which can lead to problems with liquation cracking ortemper embrittlementduring PWHT.To avoid fabrication hydrogen cracking, it is important to use low hydrogen processes and consumables, particularly as increasing the carbon and alloy content, and increasing the section thickness, increases the risk of hydrogen cracking. Apost-heattreatment may be required to reduce the levels of hydrogen in the weld region.1: Brittle Fracture SurfaceMaterials that do not fail in a ductile manner will fail in a brittle manner.Brittle fractures are characterised as having little or no plastic deformation prior to failure.Materials that usually fracture in a brittle manner are glasses, ceramics, and some polymers and metals. Under some circumstances some metals that are usually ductile will fail in a brittle manner, possibly with catastrophic results.Like ductile fractures, brittle fractures also have a distinctive fracture surface. The fracture surface of a brittle failure is usually reasonably smooth. The crack propagates through the material by a process called cleavage.The images below show the fracture surface of a steel that failed in a brittle manner.

CleavageCrack propagation (cleavage) in brittle materials occurs through planar sectioning of the atomic bonds between the atoms at the crack tip.