Cryogenic treatment benefits metals and plastics

Space race technology has come to Nottinghamshire: Cryogenic Treatment Services is using technology developed by NASA scientists to treat metals and other composite materials.

From its roots in Scotland, the cryogenic treatment of metals and plastics was developed by NASA during the space race and commercialised, through the 1990s, in the USA with many applications.

The greatest cost savings to be found have been shown when the process is applied to cutting tools.

Heat treatment implies the use of elevated temperatures, but the cryogenic process uses the below-zero range as well.

This offers significant benefits to cutting tool users.

Andy Priscott, Managing Director of Cryogenic Treatment Services, explains how the process works: "Using sophisticated cryogenic chambers that are computer controlled we can model a cooling and reheating curve down to -195C and up to +300C".

"The key to the process is the tight control of the temperature curve.

Each process requires a different curve, some remain at -195C for a number of hours and are slowly brought back to room temperature".

"Some materials require reheating to temper the material after cryogenic hardening".

Priscott continues: "In the past scientists discovered that immersing some metals in liquid nitrogen could increase their wear resistance, particularly in aircraft engines, giving a longer in-service life".

"This gave benefits, but there were disadvantages such as overly brittle material.

Thanks to NASA, developments in computer modelling and thermal insulation of the chamber, we are now able to use gaseous nitrogen".

"The tightly controlled computer process allows us to produce significant improvements to the whole structure of the materials being treated".

"In particular, we can demonstrate extended life of cutting tools that significantly reduce replacement and set-up costs for manufacturers".

Codirector, Barry Lomas, says of the science: "Cryogenics is a mixture of physics and chemistry".

"In ferrite steels, it's all about the transformation of austenite, a large soft crystal, into martensite, a smaller, harder, more compact crystal".

"And when we start to get down to -185C N-carbides start to grow throughout the structure".

"The net result is that the crystal structure is transformed with the boundary adhesion between the various crystal elements also improved, both delivering better wear properties for the treated metals".

"We can actually hold temperatures as low as -195C; hence the company's web and e-mail address are 195below.co.uk", adds Lomas.

Developing the technical argument for cutting tools, Lomas continues: "Slowly cooling a tool steel to deep cryogenic temperatures and soaking it at this low temperature for a number of hours changes the material's microstructure".

"Almost all of the austenite retained in the steel after heat-treating is transformed into a harder form, martensite, by the deep cryogenic process".

"An additional result of a deep cryogenic "soak" is the formation of fine carbide particles, called binders, to complement the larger carbide particles present before cryogenic treatment".

"This, however, depends on the alloying elements used, such as chromium carbide or tungsten carbide, in the steel".

After treatment, the cutting tool will have better wear properties.

The increase in resistance to wear occurs with the ability of the tool material to defend against a particle penetrating or gouges its surface.

This may be brought about by free grit or a foreign body in the matting surface.

The martensite and fine carbide formed by deep cryogenic treatment work together to reduce abrasive wear.

The fine carbide particles support the martensite matrix, making abrasions and scuffing of the cutting tool less likely during a cutting operation.

When a hard particle or foreign body is pressed onto the tool's surface, the carbides resist wear by preventing the particle from plowing into the surface.

Some of these benefits may be achieved through standard tempering that also transforms austenite into martensite.

However, standard tempering may not bring about a complete transformation in some tool steels.

Cryogenic treatment improves tool performance by transforming more austenite into martensite.

Overall, if an alloy contains austenite, and this austenite responds in some degree to heat treatment, further improvements will be seen after deep cryogenic tempering.

"Cryogenic processing of tooling is not a replacement to traditional heat treatment and coating techniques", stresses Priscott.

"It should be seen as a complement, an extension of heat treatment".

To encourage companies to experiment, the process is priced to be attractive.

The cost to process an average single carbide insert is GBP 1.50, drills average GBP 2.50, and the company will accept orders from GBP 75 plus VAT.

The company offers this guarantee: "If your production records clearly indicate that the processed tools show no cost advantage over the unprocessed tools, we will gladly refund the cost of the cryogenic processing".

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