The STLE Compass, released July 26, 2011

Advanced Ultra High Strength Steel (A-UHSS)
with Ted McClure, Manager, TribSys LLC in Valparaiso, IN

KARA: Hello, I’m Kara Lemar. Welcome to the STLE Compass, brought to you by the Society of Tribologists and Lubrication Engineers. The STLE Compass is your convenient and reliable resource for the latest industry developments.
This is another episode of The STLE Compass and today the focus is on advanced ultra high strength steel or A-UHSS, a topic recently featured in the July issue of TLT. Today, we borrow one of the interviewees referenced in the article and explore the content a bit further.
This advanced ultra high strength steel is revolutionizing the auto industry due to low weight, high strength and ability to absorb and repel energy. Its unique properties allow it to meet new requirements and standards, yet this material requires specialized lubrication considerations. Today, we talk to an STLE member about his experience with this material and his other work in the field.
Ted McClure, Manager at TribSys LLC, has spent over 30 years developing, testing and servicing metalworking lubricants. He currently consults and markets metalforming lubricant testing services and equipment for TribSys. He is a Past-Chairman and current Program Chair of the Chicago Section of STLE. He authored the Metalforming chapter in the STLE Tribology Data Handbook, 2nd Edition. He is a Certified Metalworking Fluids Specialist (an STLE Certification) and has served and continues to serve in a variety of capacities for STLE. He previously served as a consultant and testing contractor for the Auto-Steel Partnership Tribology team, researching the tribology of advanced high strength steels from 2001-2008, and has just recently been interviewed on his involvement with the team for TLT. We’ll get a look at this study and go further in depth on the article as we take a look at this specialized material – advanced ultra high strength steel.
Ted, welcome to STLE Compass.
TED: Thank you.
KARA: In the article, it says that you were involved in the testing of advanced ultra high strength steel. Can you tell us about the work you did with the Auto-Steel Partnership Tribology team?
TED: Sure. The Auto Steel partnership members include North American steel and auto companies organized under AISI to carry out pre-competitive research. The tribology project was funded by the Department of Energy, recognizing that AHSS is an enabling technology for fuel-efficient, crash-worthy vehicles. Auto companies were experiencing some issues forming AHSS, including springback and high die wear rates. They recognized the need for research in this area in order to be able to work with increasingly higher strength materials.
KARA: Okay. So, what kind of work did you do? And what was the result of your work?
TED: TribSys was the project contractor for laboratory friction testing. Gregory Dalton, principal researcher, and I were the TribSys representatives to the tribology team. The program was an excellent example of the diagram in the TLT article showing the progression from bench to simulation to process simulation tests.
Statistical experimental designs were used to assess the influence of various factors on the friction test results. These factors included things like the strength of the steel, steel coatings, lubricants, die materials and design parameters. The first studies used a simulation test, draw bead simulator and a bench test, the twist compression to rank combinations of factors. Based on the early studies, certain parameters were selected for further designed experiments using a process simulator, designed and built by Greg Dalton. This was essentially a very heavy duty draw bead simulator which was fitted with a decoiler allowing two to four inch wide coils of steels to be pulled through dies directly from the coils. Forces and temperatures were recorded during the tests, and die wear was measured after many pulls. Over one million strip pulls were recorded during the test program. My particular role was statistical analysis of all the data. The results formed the basis for subsequent plant trials of die materials and die failure mode analysis.
KARA: In the article, you say that there are challenges that arise from the unique properties of A-UHSS. What are some of the challenges you are referring to, and did you encounter any of those while testing?
TED: The specific challenges I was referring to are increased springback, the higher forces required to form the AHS and UHS steels and much higher die temperatures reached. The increased forces and temperatures can give rise to galling and excessive die wear. In draw bead simulator tests, one can measure the curl of tested strips to get an idea of the relative amount of springback that can be expected from steels. The higher strength steels curl more than lower strength steels. One can measure strip and tool temperatures and see the relative effect on temperature, experienced by the forming lubricants, when forming various strengths of steel. One can also see if certain lubricants are more prone to smoking at the temperatures reached. Draw beads may also be examined for buildup from the lubricant and galling. All of these can also be monitored over extended runs using the heavy duty process simulator, or die wear test.
Twist compression also provides a coefficient of friction, like the draw bead simulator, but where it really excels is in quantifying the lubricant’s ability to resist breakdown under boundary or EP conditions. One gets a time until breakdown result, which is related to the effectiveness of the lubricants to prevent galling. This compares with the draw bead test where galling assessment is strictly the qualitative absence or presence of galling observed. Unlike the draw bead, there is no lubricant wedge formed at the inlet to the contact area in twist compression testing, which feeds lubricant into the tool/sheet interface. The twist compression squeezes lubricant from the contact area continuously, producing lubricant starvation conditions. This condition is common to the failure points and high wear areas of tools in many severe metalforming operations. This is of particular interest forming high strength steels considering the higher forces involved.
KARA: You mentioned the role the lubricant had in the testing. When choosing a lubricant for use with this steel, what do you need to consider? Or can you make any recommendations to those using high-strength steel?
TED: The process of recommending a lubricant is basically the same for the advanced steels as other materials. The difference is that one must pay extra attention to areas where higher forces and temperatures might present additional challenges. Since these steels are used heavily in automotive applications, familiarity with the auto industry requirements is essential. There are strict requirements for lubricants used on the auto body, particularly in regards to downstream processes, including corrosion protection during part transport and storage, weldability, compatibility with adhesives and sealers, cleanability and electrocoat primer compatibility. Several industry test methods can be found on the auto steel partnership website under the tribology group heading. Additionally, lubricants must be able to be subjected to higher temperatures without creating strong odors or smoking excessively.
Auto body parts are typically shipped from the stamping plants to the assembly plants, often over great distances. Most of the parts have a zinc coating of some type. Lubricants must resist oxidation and protect parts from staining corrosion until finished. Exposure to higher die temperatures during forming can adversely affect the resistance of the lubricant to oxidation during transport and storage. If the oil is oxidized, this can lead to decreased corrosion and stain protection and producing a lubricant film that may be less compatible with assembly; welding and adhesives, and finishing, which includes cleaning, phosphatizing and primer application.
However, the lubricant must be able to contribute to a robust metalforming process in the stamping plant, reliably producing quality parts, without excessive die wear and downtime. Forming of parts is simulated using finite element analysis programs. This gives information on potential problem areas on the part based partially on the sheet material properties and part geometry. These modeling programs also may take into account coefficient of friction of the lubricant.
Friction testing is very important in selecting lubricants for HSS. The automotive standard for experimentally determining the COF for a given lubricant, sheet, tool combination is a draw bead simulator. However, most of the draw bead simulators in use are not heavy duty enough to draw many of the UHSS in use at the material thicknesses available. Other simulation and bench methods may be used to evaluate lubricants. It is always tricky correlating lab and shop test results. Everything that can be done to duplicate production conditions in the lab helps reduce the risk of using lab data to predict production performance in metalforming. Whatever test is used it is very important to use the same sheet and tool materials that are being used in production.
I have compared twist compression test results keeping the tool and lubricant constant and varying the sheet material between aluminum, mild steel, stainless steel, hot dipped galvanized steel, galvanneal, copper and brass. Test results varied greatly going from metal to metal. I also did a twist compression study comparing performance of a series of lubricants on mild steel compared with DP600, a dual phase HSS at three pressures. There were differences in ranking of the performance of lubricants going from mild steel to DP600 with D2 tool steel. It is not safe to assume that if lubricant A performs better on mild cold rolled steel than Lubricant B, the same will be true on HSS sheet.
We also did a large project a few years ago comparing several lubricants across several tool materials, while holding the sheet material constant. Field trials confirmed the twist compression test conclusion: that the most significant factor in this case was the choice of tool material. When this was changed, the range of effective lubricants was expanded greatly.
KARA: Given that project, what other types of work are you or your company involved in?
TED: We did some work over the last few years on biobased lubricants, sponsored by the United Soybean Board, in cooperation with Dr. Svajus Asadauskas at the Institute of Chemistry in Lithuania and Dr. Girma Biresaw at USDA in Peoria. We tested a series of conventional EP additives at concentrations between 5 and 20% in 150 second neutral basestock compared with soybean oil, using both twist compression and four ball EP tribotests. The results were very interesting in that in general it took close to twice the concentration of additive in 150 neutral oil to equal the performance in soybean oil. Additionally, we found that if 20% soybean oil was included in the 150 neutral based formulations, the performance improved to very close to that of the corresponding straight soybean oil blends. It is useful to note that many ester based lubes can withstand the heat generated in severe metalforming operations better than the equivalent viscosity mineral oil, without smoking excessively.
KARA: So what are you currently working on?
TED: With continued regulatory pressure on chlorinated paraffins, we are doing a lot of twist compression test evaluations of chlorine alternatives for additive companies and lubricant manufacturers. Another area of interest currently is heavy duty tube hydroforming lubricant evaluations, an application for which twist compression is used and accepted widely. High strength steels are being hydroformed now and conventional heat activated EP lubricants don’t seem to excel in these operations, perhaps due to the presence of a large heat sink; the secondary hydraulic fluid used to pressurize the tubes.
KARA: Given your work and your past work with the team – what kind of thoughts do you have on where this technology might be applied or where is the use of HSS headed? How are we going to be using it?
TED: I believe that in studying the metalforming system, the stronger sheet material will necessitate changes in tool materials and cost effective, lower activity lubricants will be developed which perform well with the sheet – tool combinations selected.
The main point to be made here is that if the workpiece material is changed, in this case to a HSS, one really needs to reexamine which lubricant / tool combination is most appropriate.
In forming HSS, galling and die wear are major concerns. Traditional EP additives, chlorinated and sulfurized compounds, can be effective in controlling this, but at the same time they are limited due to increased likelihood of staining or corrosion and finishing problems due to the higher temperatures and forces involved with forming HSS. There is a lot of research going on regarding the type of tooling holds up the best with the AHSS and UHS steels. Lubricants alone are unlikely to be able to address die wear issues, but they play an important role. If a coated tool is chosen to help prevent galling and wear, the need for conventional heavy duty EP type lubricants may be decreased. Some sulfurized products are not compatible with some tool coatings at all, and with less ferrous surface for the EP lubricants to react with it is not a given that they will perform well on coated tools.
KARA: Thank you Ted for joining us today and for your insight.
TED: Thank you.
KARA: I’m Kara Lemar. For more news, information and research on advanced ultra high strength steel and other automotive topics, you can visit our website, www.stle.org. Make sure you read the article to get all the information and background on this topic. Thank you for joining us today. This has been another episode of The STLE Compass, pointing you in the right direction.

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