| Steel supplier Corus is offering new grades that combine the high strength of alloy steels with the machinability of medium carbon freecutting steels. Hitenspeed 65 and 45A are intended to meet the demands for components of higher strengths but reduced processing times.
Hitenspeed 65 has a 0.2% proof stress of 820MPa, and tensile strength of 950MPa (bar diameter less than Ø40) and is intended to provide an alternative to widely used alloy steels such as 42CrMoS4. Hitenspeed 45A has corresponding strengths of 685MPa and 800MPa.
High strength alloy steels are attractive because they offer potential for mass reduction on stressed components. However, increased hardness leads to increased machining times and reduced tool life. Corus says typically, machining accounts for more than half total component cost.
In addition, these steels are usually supplied in a heat-treated condition in order to achieve their quoted mechanical properties, raising the material cost.
Hitenspeed steels are supplied as-drawn as hex or round bar, in sizes up to 70AF and Ø75 respectively.
The microstructure consists of ferrite and pearlite: there are no bainitic nor martensitic phases, whose characteristic hardness is to the detriment of machinability.
Sulphur is present at levels between 0.25-032 per cent: much higher than commonly found in alloy steels, and broadly comparable with those of free-machining steels. It aids production because it allows the steel to form chips readily during machining, even at high strength levels. The sulphur inclusions consist of manganese sulphide, whose size and shape are optimised during production by tightly controlled chemical composition and the bloom casting process.
Corus says Hitenspeed steels offer up to 200 per cent better machining performance than alternative standard materials.
Corus said: “There is a constant demand by the automotive industry to reduce costs. Significantly reducing machining time and increasing tool life has a marked effect on the component cost. Hitenspeed 65 enables us to both increase machinability and also reduce costs through the elimination of other costly processing like heat treatment while delivering a product that meets stringent mechanical properties.”
Of course, the decision to substitute one type of steel for another depends on other factors too. Loading regimes and the requirements of the application need to be considered also. Sulphur is detrimental to weld quality, and can lead to problems with porosity. Some types of fatigue loading and notch toughness may see a drop in properties, so proper evaluation and realistic life testing is important. Corus said: “Technology for testing has allowed high sulphur steels to be used in a lot more applications than they used to be. The automotive industry is finding them perfectly acceptable in a lot of applications where previously they would have shied away from.”
Hitenspeed steels are used for spindles, shafts, brake parts, components for diesel injection systems and gears. The steels are heat treatable for applications such as gears.
Corus said: “In those sorts of applications, you would consider induction hardening, so you create a surface hardness; more so than through hardening because the bulk strength is already there. You’re just looking at where you need added wear resistance to get the hardness to higher levels.”
Triumph Motorcycles is one customer. Buyer Lee Triscornia explained: “Getting material selection right plays key in product development and helps ensure we continue building unique motorcycles.” |
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