Enhanced machinability as standard

11th June 2019

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Sanmac® is the name given to a family of stainless steels produced by Sandvik with improved machinability, available as both bar and hollow bar. As Sandvik’s preferred distribution partner for duplex and super duplex stainless steels, Langley Alloys are particularly motivated by products which can make the lives of our customers easier.

 

UNS and EN standards for chemical composition allow a wide tolerance, but Sandvik products are not manufactured to deliberately target the lower end of these tolerances. Instead, they are designed to optimise the machinability, corrosion resistance and weldability. Once that target composition has been achieved, products can be manufactured with a very tight tolerances – which allows machining settings (‘speeds and feeds’) to be repeated for each job with a high degree of product consistency.

However, one of the fundamental items that Sanmac grades excel at is their favourable chip breaking quality. Chip formation is one of the most crucial aspects of any machining activity. Poorly managed chip breaking can harm the workpiece, the cutting tool and even be a major hazard for the operator. So why is chip breaking difficult when machining austenitic and duplex stainless steels, compared to other materials?

 

a) Toughness and ductility
Both austenitic and duplex stainless steels are known to have high toughness and ductility, and this is mainly due to the high elasticity of the austenitic phase structure (FCC) of the material. High ductility often leads to poor chip breaking, with long, continuous chips that can wrap around the work-piece or tool.

 

b) Low thermal conductivity
Compared to carbon steels, stainless steels have approximately a third of the thermal conductivity properties. Considerably less heat leaves the cutting process together with the chip and more heat is transferred into the insert, necessitating a lower cutting speed for stainless steel materials, compared to carbon steels. Low thermal conductivity also leads to built-up edges, where material smears onto the insert. This then rips away the coating of the insert, deteriorating the insert edge.

 

c) High proof strength
Austenitic stainless steels and normal carbon steels for machining, are quite comparable in material strength. But duplex stainless steels are about twice as strong as regular austenitic or ferritic stainless steels. Machining duplex produces strong and hard chips, which can cause chip-hammering at the insert and create high cutting forces. The high proof strength also limits both the feed rate in the material, and the cutting speed.

 

Chip-breaking, but also the energy consumption, is closely related to the microstructure. In the hot working process during bar manufacture i.e. forging and rolling, the steel has an even and optimised microstructure throughout the entire length of the bar. Non-metallic inclusions present in the steel manipulate the microstructure, so that when the material is heated up in the cutting zone, it will yield at a lower mechanical load, allowing the chip to break off easily. This will give very predictable machining, with the lowest possible energy consumption. An uneven material microstructure would directly affect chip breaking, tool-life, and result in uneven quality of the component. Sanmac materials have been designed and developed to give better chip breaking properties compared to similar products available on the market.

 

Langley Alloys stocks Sanmac grade products in the following alloys:

1. Sanmac 316L (UNS S31603 / DIN S1.4404 / ASTM A276 / ASTM A479)

Hollow Bar, from 32mm to 250mm OD.

 

2.Sanmac 2205 duplex stainless steel (UNS S31803 / UNS S32205 / F51 / F60 / DIN 1.4462)

Hollow Bar from 32mm to 250mm OD

– Solid Bar from 5/8″ to 16″

 

Sanmac is a registered trademark of Sandvik Materials Technology.