Since their introduction, maraging steels have enjoyed a reputation as the toughest of the ultra-high strength steels, at least at yield strength above 1400 MPa.

The main reasons for high toughness of maraging steel compared to that of low alloy steel of similar strength are (i) fine scale microstructure, (ii) low carbon lath martensite strengthened by a fine distribution of intermetallic precipitates rather than epsilon carbides and carbon, and (iii) type and spacing of primary particles in maraging steel and in low alloy steel.

Spacing of primary particle Ti (CN)) is about 20-40 μm which is fairly larger than those of MnS in low alloy steel with about 3 μm. The amount (volume fraction), size and distribution of Ti (CN) or TiC play a major role on fracture toughness attained by maraging steel.

In present study maraging steel is welded using Gas Tungsten Arc welding (GTAW) which is a fusion welding process and Friction Stir Welding (FSW) which is a solid-state welding process.

These two different classes of welding processes were used to see the effect of process on formation, distribution and size of primary/secondary particles on the fracture toughness of the joint.

It is observed that in GTAW process, segregation of Ti along the cell boundary promotes formation of Ti (CN). These Ti (CN) particles are coarser and closely spaced along the cell boundary which offers easy crack propagation along the cell boundary resulting poor facture toughness (K1C=110 MPam1/2).

On the other hand, there is no segregation of elements in case of FS-welded joints. Ti (CN) particles are uniformly distributed in FS-welded joints and they are finer as compared to those inherited from the primary processing of maraging steel base metal. This is one of the reasons for better fracture toughness (K1C=162 MPam1/2) property of FS-welds as compared to base metal (K1C=117 MPam1/2) and GTA-welds.

This conclusion is drawn based on the result obtained from fracture toughness test, EPMA analysis and SEM fractography of the fracture surface.

This article is shared by Suresh D. Meshram, Jalaj Kumar, Sunil Pandey and G. Madhusudhan Reddy of Defence Metallurgical Research Laboratory, Hyderabad and Indian Institute of Technology, Delhi.