The Cryogenic Treatment of Steel Components

With a whimsical nod to the “Game of Thrones” fans out there, winter is coming soon, and this October title was chosen for two reasons. One is a simple reminder to those of us that have yet forsaken proper freeze protection for pumps, pipes and other components in our systems. Second reason: Why did I really invoke the title, “Winter is Coming?” During the course of my work and due to my association with the ASM International (formerly known as the American Society of Metals) and American Society of Mechanical Engineers (ASME), I have been involved with the cryogenic treatment of steel components as it relates to the life extension and reliability of pump parts. The extraordinary low temperatures required for these processes (negative 310 F) always reminds me of winter.

Cryogenics

Cryogenics relates to the production and behavior of materials at very low temperatures. What do golf clubs, gun barrels, guitar strings, saxophones, helicopter gears, racing engines, pump parts and NASA spaceships all have in common? They can all be distinctly better when cryogenically treated. Why has “Swiss steel” been so famous for quality for the last 100 years? Is it because they post-treat the metal in a cold environment? I am not 100 percent sure, but the default (post-production) cold treatment provided by the geographical position in the cold Swiss Alps is one likely reason. Cryogenics is not to be confused with cryonics, which is the pseudoscience of freezing human bodies, most notably the famous baseball player Ted Williams. Cryogenic metal treatment has been around for approximately 100 years, but it was not until the mid-to-late 1960s that significant progress was made in the commercial processes to make it economically viable. In the science world of cryogenic treatments, it is common to use the Kelvin scale (K) for temperatures or sometimes Celsius (C), but for this article and reader familiarity we will use Fahrenheit (F). We will not use the Rankine scale even though it relates well to the Fahrenheit scale. The cryogenic temperature range is normally defined by the National Institute of Standards and Technology (NIST) as ranging from minus 238 F down to minus 460 F. Note that it is not possible to be colder than 0.0 degrees Kelvin, which is absolute zero.

Why Use Cryogenic Treatment

Not all types of metal can be treated successfully, but for the ones that can, the three main reasons are:

• Measurable improvement in wear resistance. It is not uncommon to improve wear resistance of parts by 30 percent and in some cases much more. Many steel parts (especially tool steels) experience an increase of more than 200 percent.
• Improved durability because of fewer imperfections in the steel structure. This is due to an improved and consistent grain structure in the metal.
• Stress relief. When metal cools down from the liquid (molten) state to a solid, there is always some amount of residual stress. Eliminating those stresses makes the part more reliable because mitigation of the residual stress will result in a direct reduction of fatigue failures.

The cryogenic treatments also improve corrosion resistance as the process offers some added protection from acids and caustics. Additionally, there is added resistance to abrasion experienced in some slurry applications. One cryogenic treating firm also purports added protection from cavitation damage. Further, when metals are properly treated cryogenically, it is not just a surface (substrate) fix. The process effect typically occurs throughout the entire piece. One of the reasons why tool steel manufacturers and users love this process is the tool can be ground or resharpened countless times because the process is not just a coating or a few mils thick. As a real world example: the functional life of slitter knives used at paper mills can be increased as much as 600 percent. Cryogenic treatment of metals can be defined under many commercial names and processes. For example: cryogenic hardening, cryogenic processing, cryogenic tempering, deep cryogenic treatment (DCT), and variations on all of these. As in any case when dealing with vendors, please be wary of marketing claims. Look for experience, formal technical procedures, real science (data and facts) and proven results. The good processors will typically have a laboratory or a professional connection and/or access to one, for the purpose of data proof/support. Different companies have different proprietary processes, temperatures, durations and techniques, and so the results may vary and have other descriptors. Various claims for different metals from different processors include statements of increased hardness, enhanced toughness, better wear resistance and dimensional stability. End users will need to work with the vendor chosen for specific information. With apologies to my engineering and metallurgist associates: For the general discussion in this article, we will describe cryogenic treatment simply as a slow and regulated cooling process of pieces/components that were previously heat treated, followed by a subsequent holding period of approximately 24 hours, then followed by a controlled return to ambient temperatures. Actual hold times may vary from four to 48 hours, and specific times and temperatures will depend on the geometry and material of the parts being processed. Sometimes, the process involves reheating the piece above ambient after the cold treatment. The cooling process for cryogenic treatment is usually accomplished by a gaseous process using liquid nitrogen (LN₂) technology. My statement of gas and liquid in the same sentence seems in conflict, but to explain, they use LN₂ and expand it to a gas through a valve to accomplish the cooling effect. This procedure is a throttling process or it is also known as the Joule-Thomson process. Other gases used in liquid form such as liquid helium (LHe), liquid oxygen (LOX) and liquid hydrogen (LH₂) may be used, but due to the high cost of these other gases, nitrogen is the most commonly used. Please be advised if all that is involved in the process is dry ice (solid CO₂), it is simply a cold treatment at approximately minus 100 F. This CO₂ treatment is often referred to as shallow cryogenic treatment (SCT), and it is technically not cryogenic treatment, which would be defined as starting at minus 238 F. However, not all metals or processes require the DCT. Some companies may also use the phrase “cryogenic tempering,” but please realize this is technically a misnomer and more of a marketing phrase, albeit a catchy one. One of the main issues and a potential obstacle to general acceptance of this technology is the mindset that items dropped in LN2 become brittle and can break easily. We have all seen the science experiments in classroom labs or on TV shows that perpetuate this paradigm. According to T. Yugandhar and associate P.K. Krishnan in a technical paper they published on steel parts and tools used in the nuclear fuel industry: “If traditional tempering practices are followed the potential advantage of deep cryogenic treatment may not be realized and the net effect on the metal properties could be negative.” Another caveat is just like the computer programing mantra or cliché of “garbage in, garbage out,” you must start with a good piece of metal before the process begins. So, what we do know is that an uncontrolled and rapid dip in LN₂ will make objects hard and brittle, but when you control the rate of temperature change and the duration (hold points) at specific temperatures, you can achieve significant and positive effects. The process cost can easily be justified due to the measurable return on investment.

For the Metals & Technical Folks

Steels can appear in many microscopic structures and forms that include:

• ferrite (also known as alpha iron)—a body-centered cubic structure
• austenite (also known as gamma iron)—a face-centered cubic structure
• martensite—a steel structure obtained only when austenite is suppressed down to a temperature where it is forced to transform to a body-centered tetragonal

That specific process, by the way, is why these cryogenic processes are successful. The fundamental point of all this technical information is what happens at ultra-low temperatures to the austenite steel structures as they transform to martensite. The cryogenic process changes (transforms) the ever-present and residual austenite to martensite in the steel. This transformation is usually, but not always, a desired outcome in the metal properties, and so treatments of common-duplex and super-duplex metals like CD4MCu and other austenitic steels may not be viable or recommended. End users should work with a knowledgeable vendor to fully explore the benefits, limits and consequences of the process. Cryogenic processes should not to be confused with precipitation-hardened (PH) steels like 17-4 PH steel. These steel alloying processes involve an extended period of time at high temperatures, which is what allows the precipitation process to take place. In the precipitation process, some formula or combination of molybdenum, copper, titanium and/or aluminum are alloyed with the steel. All of the key process steps take place at high temperatures, not at cryogenic levels. Although, you may witness that sometimes 17-4 PH steels are post-process treated in either cold and or cryogenic environments. I also understand that cryogenic processes may be applied to metals other than steel, such as copper, aluminum and titanium, but I have little to no knowledge or experience with those materials. Of further interest is that cryogenics has also been successfully used on some types of plastics.

Cryogenics for Pump Applications

We all have experienced bad late-night drama with a “problem child pump” that somehow always defies an economically feasible solution. Either the flow is away from best efficiency point (BEP) or the head is too high, the pH is lower than anticipated or the solids and the consequential abrasion are too aggressive. Not to mention the one big physics problem we all eventually run into, which is “nobody makes a pump that can handle net positive suction head available (NPSHa) at such a low value” conundrum. No matter your actions, the pump is short lived. Solutions, if there are any, for these application problems are frequently expensive, time-consuming and difficult because they require highly engineered pumps that are special or out of reach. The combination of these issues contributes to exorbitant costs. Perhaps instead of trying to solve the root problem, we simply treat the symptoms? Maybe, for a 20 percent premium on the current pump parts, we can double or triple (and often more) the life of the components. That positive result of extended life and increased reliability is what cryogenic treatment can potentially bring to the value proposition. Cryogenics is not the solution to all of your pump problems, but in many cases in the middle of the cold, dark night, it can be the protection from the storm that you are so desperately looking for. And don’t forget—winter is coming. Want more information? Reference the Cryogenic Society of America website for additional information, especially the Cryogenic Treatment Database on that site. Also reference the ASM Handbook Volume 4a.