TWI Frequently asked questions
The role of niobium as an alloying element in steel is due to both its effect in solution and also, its ability to combine with carbon and nitrogen to form fine precipitates, whereby it effectively increases the steel strength through the following mechanisms:
- Grain refining;
- Retardation of recrystallisation;
- Precipitation strengthening, and;
- Retardation of austenite decomposition
The response of niobium microalloyed steels to welding is complex due to the interaction of the above referenced mechanisms. As with other microalloyed steels, when niobium microalloyed steels are welded the toughness of the parent material may be reduced in the Heat Affected Zone (HAZ) due to the formation of low temperature transformation products and precipitation of fine particles. However, this effect is also dependent on other aspects of the parent steel chemical composition, steel thickness, and welding parameters.
It is well known that the HAZ toughness is also highly dependent on the carbon content of the parent steel. Typically, steels with a lower carbon content (0.04 to 0.07%) exhibit better toughness and even more so with lower bainite or autotempered martensite microstructures.[Graf and Niederhoff, 1990] To compensate for the loss of strength when using lower carbon contents, microalloying additions are generally made together with the practice of thermo-mechanical (TM) processing.
The influence of niobium and factors determining HAZ toughness has been well summarized in papers by Kirkwood[Kirkwood, 1981 and 1987] and Batte et al.[Batte et al, 2001] It is important to remember that the HAZ itself comprises of a wide range of microstructures, each influenced by the heating rate, peak weld temperature and subsequent cooling rate during weld deposition and solidification, as well as the original hardenability (chemical composition) of the parent material. For multi-pass welding the picture becomes even more complex as the HAZ from the second bead overlaps the HAZ from the first bead, creating an intercritically reheated grain coarsened HAZ.
Depending on the content and parent steel chemistry, niobium can influence the coarse grain HAZ microstructure in several ways due to the solubility characteristics of its carbonitride precipitates i.e. being more soluble than titanium but less soluble that vanadium and molybdenum.[Batte et al, 2001] If the time at peak temperature, or at a level at or above the solubility of the niobium carbonitride (Nb(CN)) precipitate, is sufficient then dissolution will occur permitting the austenite grain size to grow. Conventionally this is prevented via small additions of titanium which form stable nitrides that pin the austenite grain boundary, preventing growth. As the weld area cools a degree of re-precipitation may occur in austenite and or ferrite which can restrict grain growth and may promote early nucleation of ferrite at the boundaries. At intermediate cooling rates the solute niobium will decrease the transformation temperatures,[Abe et al, 1985] upper bainite and martensite formation may increase and martensite-austenite (M-A) islands may form under certain conditions. The development of such M-A phase is known to lower the toughness in the intercritically reheated HAZ of microalloyed steels[Shiga, 1990] typically at volume fractions above 4%.[Akselsen et al., 1988] However, the morphology of the developed M-A phase is also an important factor with blocky phases being more detrimental.[Suzuki et al., 1996] To some extent, the development of M-A phase can be limited by control of welding procedure.
Nb-microalloy additions of up to 0.10%Nb have successfully been made to generate excellent strength and toughness in the parent material for commercial large diameter gas pipeline projects.[Yuggun et al, 2008, Stalheim, 2007] Nevertheless, as with other microalloys, the niobium content must be carefully controlled to ensure that precipitates developed during welding are neither too coarse nor too fine, whilst keeping in context the overall steel chemical composition, steel thickness and welding parameters applied.
As mentioned earlier, the effect of niobium on HAZ toughness is strongly dependent on the heat input.[Shiga, 1990, Mitchell, et al., 1995, Barnes, 1990] At low heat inputs (~2kJ/mm), as used for pipeline girth welding, niobium has little, or no deleterious effect on as welded properties[Shiga, 1990, Mitchell, et al., 1995, Barnes, 1990] as cooling rates are very rapid especially for thicker steel plates. Even at levels approaching 0.10%Nb with a low carbon, inclusion shape controlled, low-alloyed base steel, excellent HAZ properties can be achieved.[Fazackerley et al., 2007, Graf et al, 2002]
At higher heat inputs, to allow economical welding of thicker sections or plates, or to avoid over hard HAZs in higher carbon and alloy containing steels, the tolerable niobium limit is progressively reduced to 0.02-0.05%Nb. This is because there is increased opportunity for the precipitation of undesirable phases and grain growth. This only further underlines the point that the effect of niobium on the HAZ properties is also influenced by other factors which must be taken into consideration.
For extremely high heat input welds, a high excess of solute niobium is known to favour upper bainite formation and also M-A phase, and therefore the niobium contents are limited for some steel applications. However, decreasing the carbon content and or other elements such as silicon will greatly help the decomposition of the M-A phase during any subsequent heat treatment cycle. When the heat input is very high, >50-60kJ/mm, low niobium levels (0.015%Nb) are typically required alongside low carbon levels. Here it has been found that the influence of the austenite grain growth on the HAZ toughness is also diminished and the HAZ microstructure itself becomes the dominant factor controlling the toughness.[Suzuki,et al., 1980]
For some welded structures, especially involving thicker plates and sections, a post weld heat treatment (PWHT) is required to reduce the residual stress in the weld region. During this heat treatment process, any soluble niobium may precipitate on existing particles and coarsening may occur via solute niobium as well as by Ostwald ripening. Alternatively, precipitates may nucleate at favourable locations and thus increase the HAZ hardness. Again, depending on the basic microstructure of the HAZ, hardening can result in notch toughness reduction. This hardening effect may be mitigated by concurrent tempering of the hard, brittle phases within the microstructure. Although the toughness of the HAZ drops after PWHT to a greater extent with higher niobium content[Kirkwood, 1981 and 1987] it is more so during slow cooling conditions. For stress relieving conditions there are many other factors that will also play a role[Kirkwood, 1981 and 1987] and excellent results have been obtained even when the parent plate niobium content is as high as 0.06%Nb.[Garland and Kirkwood, 1974]
References
- M.Graf and K.Niederhoff: Toughness behaviour of the HAZ in ductile submerged-arc welded large diameter pipe. Pipeline Technology Conference, 15-18 October 1990, Ostenend, Belgium
- P.R. Kirkwood: Welding of niobium containing microalloyed steels. Niobium science and technology, Proceedings of an International Symposium Niobium 1981, TMS, USA
- P.R. Kirkwood: Welding of HSLA steels - a perspective. Proceedings AIME International symposium on welding metallurgical structural steels, Denver, USA, June 1987, pp.21-44.
- A.D. Batte, P.J. Boothby and A.B. Rothwell: Understanding the weldability of niobium-bearing HSLA steels. Niobium science and technology, Proceedings of an International Symposium Niobium 2001, TMS, USA,pp.931-958
- K. Abe, K. Tsukada and I. Kozasu: 'Role of interrupted accelerated cooling and microalloying on weldable HSLA [high strength low alloy] steels'. HSLA Steels: Metallurgy and Applications. Proceedings,International Conference, HSLA Steels '85, Beijing, China, 4-8 Nov. 1985. Eds: J.M. Gray, T. Ko, S.H. Zhang, B.R. Wu, X.S. Xie. Publ: Metals Park, OH 44073, USA; American Society for Metals (ASM) International, 1986.
- C. Shiga: The metallurgy, welding and qualification of microalloyed (HSLA) steel weldments. Houston, November 1990, p.343
- O.M. Akselsen, J.K. Solberg and O. Grang: Effects of MA-islands on intercritically HAZ toughness of low carbon microalloyed steels. Scandinavian Journal of Metallurgy, 17, 1988, pp.194-200
- S. Suzuki et al: Review of mechanical and metallurgical investigations of MA constituent in welded joints in Japan. Welding in the World, 37(3), 1996, pp.134-154
- Y. Yuggun et al: The Development of X80 steel plate and coil for the 2nd West-East pipe line project. Proceedings of IPC 2008, Sept 29 - Oct 3, 2008, Calgary, Alberta, Canada
- D.G. Stalheim: The use of high temperature processing (HTP) steel for high strength oil and gas transmission linepipe steels. Proceedings of microalloy steels for the oil and gas industry symposium, TMS, 2007, pp.73-108
- P.S. Mitchell, P.H. Hart and W.B. Morrison: The effect of microalloying in HAZ toughness. Proceedings of the International Conference ‘Microalloying '95, Iron and Steel Society, Pittsburgh, PA, USA, June 11-14, 1995,pp.149-162
- A. Barnes: Local brittle zones in C-Mn steel multipass welds. TWI Bulletin, September/October 1990
- W.J. Fazackerley, P.A. Manuel and L. Christensen: First X80 HSLA pipeline in the USA. Proceedings of microalloy steels for the oil and gas industry symposium, TMS, 2007, pp.353-366
- M. Graf et al: Production of large diameter pipe grades X70 with high toughness using acicular ferrite microstructure. International conference on application and evaluation of high grade linepipe in hostile environments, Nov 8-9,2002, Yokohama, Japan
- S. Suzuki, T. Kamo and Y. Komizo: Influence of martensite-austenite constituent o the HAZ toughness of a high strength steel, IIW Doc.IX, 1980, pp.1156-80
- J.G Garland and P.R. Kirkwood: The notch toughness of submerged arc weld metal in micro-alloyed structural steels, IIW Doc. IX-892-74, June, 1974
