■ By Pete Paulin

Although its effects on metal

composition are subtle,

deep cryogenic tempering can yield dramatic improvements

in tool performance.

nn the search for cutting toolengineering that can increase productivity, prolong cutting

life, and decrease costs, gains of 15% to 20% are considered significant. One re­cently developed tool treatment is show­ing far greater promise, in some cases improving tool life by 200% to 400%.

The method, called deep cryogenic tempering, subjects tools placed in a specially constructed tank to tempera­tures below -300° F for a number of hours using liquid nitrogen as the refrig­erant. The process supplements stan­dard heat/quench tempering, complet­ing metallurgical changes that heat treat­ing begins.

Since 1965, when commercial deep cryogenic treatments first became avail­able in the United States, a handful of studies and reports have been released noting the improved performance of treated lathe tools and of tool steels used in the steel industry. According to the literature, some machine elements, such as progressive dies used in metalwork­ing, have lasted six times longer after deep cryogenic treatment. Drills, end-

mills, and taps also have shown signifi­cant improvement.

Another advantage of deep cryogenic tempering revealed through research is its ability to change the entire structure of the tool material, not just its surface. As a result, the treatment is not negated by subsequent finishing operations or regrinds.

Cool and ControlledFor all its advantages, however, deep

cryogenic tempering is no panacea. In some cases it has produced sterling re­sults: One manufacturer of titanium- alloy parts reports that, after treating the

M-42 twist drills it uses, the company needed 63% fewer of the tools to do the same work. In another instance, a 400% improvement in tool durability was achieved using cryogenically treated C- 2 carbide inserts to mill 4340 stainless steel. However, other metals, such as T- 2 tungsten HSS, have been left with little or no change after treatment. Even where deep ciy ogenic treatment has been shown to be effective, results have not been consistent.

But as researchers and commercial applicators have learned more about the deep-cryogenic process, they have dis­covered ways to produce more repeat-

T E S T RESULTS: Percent of Increase in Wear Resistance After Cryogenic Tempering

AISI# Description At -110° F At -310° FD-2 High carbon/chromium die steel 316% 817%S-7 Silicon tool steel 241% 503%

52100 Standard steel 195% 420%0-1 Oil hardening cold work die steel 221% 418%

A-10 Graphite tool steel 230% 264%M-1 Molybdenum high-speed steel 145% 225%H-13 Chromium/moly hot die steel 164% 209%M-2 Tungsten/moly high-speed steel 117% 203%T-1 Tungsten high-speed tool steel 141% 176%

CPM-10V Alloy steel 94% 131%P-20 Mold steel 123% 130%440 Martensitic stainless 128% 121%

M aterials that did not show significant improvement

430 Ferritic stainless 116% 119%303 Austenitic stainless 105% 110%8620 Nickel-chromium-moly alloy steel 112% 104%

C-1020 Carbon steel 97% 98%AQS Graphitic cast iron 96% 97%T-2 Tungsten high-speed steelR.F. Barron study results, Louisiana Polytechnic Institute

72% 92%

The effects o f shallow and deep cryogenic treatments on various steels are shown. Percentage increases above 120% are considered statistically significant.

CUTTING TOOL ENGINEERING AUGUST 1 9 9 2 /6 1

able and predictable effects. The trick is In the precise management of the cool- ing sequence. The older tanks did not adequately control temperatures as they were brought to deep-cryogenic levels. Today’s state-of-the-art deep-cryogenic systems use a computer linked to the tank that duplicates the optimal cooling curve. It carefully regulates the tem­perature change and brings a measure of consistency to the process.

Under a computer’s control, tempera­tures inside a cryogenic tank are brought

6 2 /AUGUST 1992

down according to a prescribed time­table. The process unit manufactured by 300° Below Inc., Decatur, IL, cools the material slowly to -317° F, holds the temperature at that point for 10 to 40 hours, then raises it to +300° F before slowly returning it to room temperature. It is a dry process that, unlike other deep cryogenic processes, does not bathe the materials in liquid nitrogen, a practice that the designers of 300° Below’s unit believe is more likely to cause damage from thermal shock.

Chilly ChangesSlowly cooling a tool steel to deep

cryogenic temperatures and soaking it at this low temperature for several hours changes the material’s microstructure. Almost all of the austenite (a soft form of iron) retained in the steel after heat treating is transformed into a harder form, martensite, by deep cryogenic tem­pering.

A second 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. (Depending on the alloying elements in the steel, these particles might be chromium car­bide, tungsten carbide, etc.)

One recent study by Randall Barron, Department of Mechanical Engineer­ing, Louisiana Polytechnic Institute, Ruston, LA, looked at how the changes brought about by cryogenic treatment affected steel’s ability to resist abrasive wear. This type of wear occurs when a body penetrates and gouges a material’s surface. The gouging body may be a surface asperity on a mating part, a free- abrasive grit particle from an external source, or an internally generated wear particle.

The Barron study found that 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 it less likely that lumps will be dug out of the cutting tool material dur­ing a cutting operation and cause abra­sion. When a hard asperity or foreign particle is pressed onto the tool’s sur­face, the carbides further resist wear by preventing the particle from plowing into the surface.

Some of these benefits may be achieved through standard tempering, which also transforms austenite into martensite. But standard tempering may not bring about a complete transforma­tion in some tool steels. For example, 8.5% of an O- l steel remains austenite after it is oil-quenched to 68° F. If M-l is quenched from 2228° F to 212° F, then tempered at 1049° F, the retained auste­nite is 11%.

Additional improvements in tool per­formance can be achieved if this re­tained austenite can be transformed to martensite. As Barron’s study has con­firmed, adding a cryogenic step to the treatment process does just that.

In the chart accompanying this article, data drawn from another study of treated metals by Barron indicates which samples exhibited improved abrasive

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cryogenics

wear after cryogenic treatment. In addi­tion to results obtained from samples treated at liquid-nitrogen temperature (-310° F), the chart also lists results of treatment at dry-ice temperature (-110° F).

Predicting EffectivenessKnowing how deep cryogenic tem­

pering works, we can predict which materials will benefit most from treat­ment. Generally, if an alloy contains austenite, and this austenite responds in some degree to heat treatment, further improvements will be seen after deep cryogenic tempering. For instance, fer- ritic and austenitic (430 and 303) stain­less steels generally cannot be hardened by heat treatment. Martensitic (440) stainless steels, on the other hand, can be hardened by heat treatment. Therefore, the effect of deep cryogenic treatment should be more pronounced for440 stain- less steel than for the other stainless steels.

C-1020 carbon steel and QS Meehanite iron also show no significant improve­ments in performance after cryogenic treatment. Because these materials con­tain no austenite, sub-zero temperatures can cause no further metallurgical changes in them.

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cryogenics

CriticsIn addition to examining cryogenic

treatment’s effects on austenite trans­formation, Barron also looked at other, less obvious metallurgical changes. His data helps answer those critics of cryo­genic tempering who doubt the effec­tiveness of a process that imparts so few visible changes to the metal. These crit-

6 6 / AUGUST 1992

ics say heat treating already changes 85% of the retained austenite to marten­site, leaving only 8% to 15% to be trans­formed by deep cryogenic tempering. Using the process to produce such small results is inefficient, according to the skeptics.

These observations are accurate as far as they go. But they fail to take into account the fact that metals subjected to deep cold develop a more uniform, re­fined microstructure with greater den­sity. The particles that are formed through

the precipitation of additional microfine- carbide fillers take up the remaining space in the micro voids, resulting in a tool steel with a much denser, coherent structure that improves wear resistance.

Metallurgists have known that cryo­genic tempering has this effect, but they may not have realized the extent to which the microstructure changes. Research­ers in a study conducted at the Jassy Polytechnical Institute in Rumania used a scanning electron microscope equipped with an automatic particle counter to identify and quantify these smaller par­ticles. Through this examination they found that cryogenic tempering creates a significant change in density through­out the tool.

Some people acknowledge the ben­efits of cold tempering, but they ques­tion the need to use temperatures below -110° F. A review of the data from the Barron study, however, shows that some tools, at least, perform significantly bet­ter after processing at -310° F than they performed after -110° F tempering. Among cutting tool steels, for instance, the wear resistance of M -l HSS was almost one-and-a-half times the wear resistance of the untreated material after a -110° F soak. After a -310° F soak, the steel exhibited two-and-a-quarter times the wear resistance of the untreated material. T-l, the traditional tungsten HSS, went from a little under one-and- a-half times the wear resistance after a -110° F soak to one-and-three-quarters the wear resistance after a -310° F treat­ment.

It may be concluded from these tests that treatment at -110° F does improve wear resistance, but deep cryogenic treat­ment at -310° F improves wear resis­tance much more. To allow time for the very fine carbide particles to form and the retained austenite to be transformed into martensite, a long soak at deep cryogenic temperatures is necessary.

SummaryDeep cryogenic treatment has been

shown to result in significant increases in the wear resistance of steels such as D-2, M-2, O-l, and 52100. The basic mechanisms at work during the cryo­genic process help control wear by pro­ducing a tough surface, which helps to prevent particles from tearing out of the material and resist penetration of the surface by other particles. A

About the AuthorPete Paulin is president o f 300° Below Inc., Decatur, 1L.

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