TWI Technology Briefing 628 - October 1997
A M Barnes
FULL REPORT
Modification of a standard 9%Cr1%Mo composition, through additions of Nb, V and N, has led to evolution of a steel grade that offers good high temperature strength, oxidation resistance and resistance to hot hydrogen attack.
Background
This material is finding increased use within the power generation industry, for example in heater and heat exchanger tubing, and its potential is acknowledged in other industry sectors. Development of consumables began with matching compositions, but this led to weld metals of inferior properties relative to the parent steel. It was found that the levels of C and Nb were critical in the determination of toughness, but that some Nb was required to ensure adequate creep strength. Whilst creep properties used in high temperature applications, toughness is a consideration if start up/shut down problems are to be avoided, and pressure test requirements satisfied.
Many studies at TWI and elsewhere have shown that modification of the current commercial consumable compositions can lead to significant enhancement of properties, particularly toughness. A variety of elemental effects has been investigated, and the present work examined the effects of Mn and N on deposit microstructure and on both Charpy and CTOD toughness.
Objective
- To determine the effect of manganese and nitrogen on weld metal hardness and toughness and on microstructural development.
Experimental approach
Three multipass manual metal arc welds have been made at an arc energy of 2 kJ/mm using experimental electrodes incorporating controlled variations in Mn (0.69-1.47%) and enhanced N (639ppm). The welds were made in samples of 25mm thick high strength carbon manganese steel buttered with modified 9%Cr1%Mo weld metal to ensure that deposits of low dilution were produced. Microstructural examination, including a point counted assessment of delta (δ)-ferrite content, and hardness testing were carried out for each deposit in the as-welded condition and following PWHT of two hours at 760°C. Charpy and CTOD transition data were generated for the PWHT condition. Charpy testing of cap and root subsurface locations was carried out; the CTOD transition data were generated using full thickness specimens.
Main conclusions
From the results generated, the following conclusions can be drawn for welding and PWHT condition and compositional ranges used.
- A good indication of weld metal microstructure can be obtained from the chromium equivalent and Kaltenhauser ferrite factor. The microstructure, as evidenced by optical micrography, is fully martensitic for Cr eq <8and FF <6.
- The compositional variations studied did not produce any significant modification to the essentially fully martensitic weld metal microstructure.
- The variations in weld metal composition studied did not give rise to significant differences in the Charpy toughness of the deposits, but CTOD testing indicated a detrimental effect of high nitrogen (~640ppm) and decreased Mncontent (~0.7%) after PWHT of 2 hours at 760°C.
Recommendations
This study has indicated a detrimental effect of reduced levels of Mn (0.69%) and enhanced nitrogen (~640ppm) on fracture toughness. In light of this, and earlier work, for applications where toughness is of concern, consideration should be given to use of weld metals with enhanced Mn levels (up to ~1.5%) but with levels of nitrogen of ≤500ppm and with levels of Si of ≤0.2%. Nb of ~0.04%. The addition of ~1.0%Ni may also be beneficial, although the effect of the combined Mn and Ni additions has not been studied. It should, however, be noted that the creep properties of deposits in the present study have yet to be established.
Member Report No. 628-1997
The effect of manganese and nitrogen on microstructure and toughness of modified 9%Cr1%Mo weld metals
