Cryogenics: Putting the freeze on wear

Cryogenics is a branch of physics dealing with subzero, or deep-cold, treatment.

Cryogenic treatment of metals is not a new process. However, its use as a method to improve tool wear characteristics is still considered to be a relatively new engineering field. The cryogenic process produces considerably longer wear life for most heat-treated tools or parts that are subject to wear and abrasion.

The key factor for a stamping company or stamping tool expert is that this process should extend tooling life and greatly reduce the need to resharpen dies and downtime.

The following are some areas to explore if you are considering using cryogenics.

How does cryogenics work?

For heat-treated ferrous metals, cryogenics completes the heat-treating process. It continues transforming the metal's grain structure (retained austenite) into a usable, life-extending structure (martensite). Although carbides are not heat-treated ferrous metals, cryogenics affects their bonding materials, causing a tighter grip on the carbides and thus preventing premature carbide losses, called fluffing off.Cryogenics also is a stress reliever. Although often forgotten and not taught in tech schools today, subzero cold is an effective stress reliever for ferrous and nonferrous metals. The process doesn't discolor or cause oxidation on the surface of the metal.

Can existing stamping dies be processed cryogenically?

Using cryogenics on existing dies is not a good idea. Whenever a ferrous metal is heat-treated and cryogenically processed, it grows in size. The growth can throw stamping dies far enough out of tolerance to make them unusable.

If you want to gain the extended wear that cryogenics can offer, cryogenically treat the existing die components after heat-treat and tempering and before final grinding or electrical discharge machining (EDM). Be aware that dowel pinholes, or any features added before processing, may not line up after processing. The amount of growth is not predictable but will be most significant in poorly heat-treated parts and less significant in properly heat-treated parts. But be assured they all will grow.

Once tooling has been cryogenically treated, subsequent cryogenic treatment used for nondiscoloring stress relief will not cause growth, unless the component is annealed and reheat-treated for design changes.

Can higher-cost tool steels be replaced with lower-cost, easier-to-machine tools that have been cryogenically processed?

This is entirely possible for some applications. However, it is very hard to change the solid rules of selection.

Every application must be evaluated separately. The parameters then can be evaluated to choose the right steel grade for that specific application.

Careful selection of the metal grade and heat-treatment procedure will produce the optimal metal properties.

It is quite common for toolmakers to make a part from, for example, A2 tool steel and then temper the part to 54 to 56 Rockwell hardness C to add toughness. They should instead consider using a shock-resistant steel such as S7 to do the job.

Tool designers and makers need to understand the importance of using the right metal, based on the parameters of the application. It's vital to get all the facts, know about the tool steels, and make educated choices. Don't use a steel because Gramps did it that way or because it's the only steel in stock.

Will the life of the die be more predictable?

The life of a cryogenically processed die is not more predictable than an unprocessed die. A tool's increased wear resistance can't be predicted accurately, either by grade or by application. The controlling factor still depends greatly on how well the heat-treatment process was performed.

On occasion, some cryogenically treated tools don't show wear resistance increases because the manufacturers treat their tools after heat treatment. Others use cryogenics to correct bad heat-treatment processes.

Do tools get brittle after being processed?

Tools will not get brittle after being processed if they are tempered after cryogenics is performed and are temper-soaked long enough to stabilize the fresh martensite formation. The posttemper needed should be only 300 degrees F for two hours for each inch of thickness.

Processing companies may stack tools up during processing, causing the tools to be only partially tempered on the outer edges of the stack. Many poorly made processors have onboard heating elements used to temper the load after cryogenic processing. This is the main contributing factor to lack of air space around the parts and uneven tempering of the entire load. Plus, onboard heat often creates oxidation on the surface, which ruins the immediate effect of increased wear resistance until the tool is resharpened.

Can cryogenic processing be performed in-house?

Yes. Small, easy-to-operate processors are on the market for use in the tooling department or an in-house heat-treat operation. These processors have programmable controllers that run the operation from start to finish, using your own tempering furnace for the final critical step.

What should I know before using a cryogenic process?

Cryogenics is not a cure-all. For heat-treated ferrous metals, cryogenics is simply the continuation of the heat-treat process. It can increase wear resistance by transforming austenite to fine-grained martensite. Plus, if the alloy and carbon content are supportive, a profusion of fine carbides will be precipitated, yielding increased wear.

Caveat emptor! If you're about to get involved in cryogenics, investigate the company you're considering using.

If the process will be done in-house, first view the application. If a stamping die is wearing too fast or breaks often, find out why. It's just like being a detective or a doctor. It's imperative to make an accurate diagnosis before prescribing a treatment. Look at the heat-treatment process that was used. Was it the right recipe? Was the recipe followed, or was a short cut taken?

Simply put, cyrogenics can improve the life of your tools, but make sure to ask the proper questions before making a choice.