Tougher steels for inverse temperatures

Materials World magazine
,
1 Aug 2008

Researchers at the National Institute for Materials Science, in Tsukuba, Japan, claim to have developed a low-cost, ultra-high strength, low-alloy steel with a Charpy V-notch impact energy (vE) six times greater than highly-alloyed maraging steels that have a yield strength of 1.8GPa at room temperature. They say the metal could be used in applications such as bolts and shafts in low-temperatures where ductile-to-brittle transitions occur.

The scientists believe the combination of ultragrain refinement of a low-alloy steel (0.4% carbon, two per cent silicon, one per cent chromium, one per cent molybdenum, and the balance iron), dispersion of carbide nanoparticles and novel processing conditions control texture and grain shape can lead to ultra-high strength with enhanced toughness as well as ductability in body-centered cubic steels at lower temperatures of down to -100˚C.

Traditionally, low-alloy steels with ultra-high yield strength exceeding 1.4GPa exhibit a low Charpy vE of 10-40J at room temperature. Stronger, tougher maraging steels, containing large amounts of nickel, cobalt and nanometer-size precipitates, are often used to overcome this problem. However these are costly and still only have a vE of 40J at room temperature with ultra-high yield strengths over 1.8GPa.

‘Low toughness often restricts [the use of high-alloy steels] in certain structural applications, hence, high toughness has been zealously pursued at low-temperatures’, says Dr Yuuji Kimura, lead researcher on the project.

Tempforming

Contrary to conventional quenched and tempered steel with a random crystallographic orientation, the new sample has an ultrafine elongated ferrite grain structure with a strong deformation texture. This is interspersed with nanometre-sized carbides to suppress grain coursing during deformation, and strengthens the steel.

‘The refinement of crystalline grain size is key for lowering the ductile-to-brittle transition temperature for high-strength metals, but until now, reducing the grain size to below one micron resulted in low toughness and ductility at room temperature,’ explains Kimura.

‘High-strength steels are conventionally worked and formed at room temperature, after softening through annealing. However, there is a trade-off between softening and strengthening of the material, hence cold forming becomes difficult as the strength of the material increases,’ he adds.

The technology in Japan resolves this problem and eliminates the need for annealing. The researchers treated a four-centimetre-thick block of steel at 1,200˚C for one hour before hotrolling it into a square bar which was then cooled to 500˚C. It was then deformed with a multipass calibre roller into the finished bar with the fibrous grain shape – a process they call tempforming.

However, a UK industry representative told Materials World that he is sceptical about the research. ‘The term “tempforming” seems to be a new name for “warm forging”, which is well known and [can be expected] to result in the directionality and elongated ferrite grains,’ he says. ‘Likewise, the nanosized particles will act as fine precipitates of carbide, but this is usually referred to as secondary hardening, something that has been well known since the 1950s.’

He adds, ‘There is interest in the effect of thermomechanical treatment of low alloy steels and the properties of ultrafine grain structures, but at present this is confined to laboratory studies and I can see the relevance of the work in this context. However massive investment is needed for someone to make this technology commercially viable and it does not seem revolutionary enough to attract this’.

Kimura says, ‘The evolution of ultrafine fibrous grain structures has been [previously] confined to thin wires such as piano wire, because severe plastic deformation is usually required through means such as cold drawing and swaging. Our idea for the production of an ultrafine fibrous grain structure is to deform tempered martensitic structures at 500˚C’.

He acknowledges that the ‘tempforming’ process more expensive than the conventional quenching and tempering for low-alloy steel, but his team is looking to modify the method for large scale industrial production.

The group is also working to create an ultra-high strength bolt and establish a mechanical fastening system for high-strength materials as an alternative to welding.