Case Studies

The Gleeble at Rensselaer Polytechnic Institute

At Rensselaer Polytechnic Institute (RPI), a profile of research involving the Gleeble is a study in contrasts. Some applications reflect the original reason why the Gleeble was developed. Others present a radical departure from traditional thinking about the Gleeble.

But that's hardly a surprise. It was at RPI in the 1950s that a graduate student and two faculty members worked to solve a vexing problem: how to simulate the heat-affected-zone of a weld. They devised a technique to accurately control the temperature of the specimen while power for resistance heating is being applied.

The idea worked so well that, in 1957, they founded a company to manufacture a machine for thermal metallurgical studies — and the Gleeble was born. Over the years, the Gleeble has evolved to include a hydraulic servo-mechanical system for dynamic thermal-mechanical studies, a computer control system, and a powerful data acquisition system.

But despite advances in materials design and materials testing, some problems remain the same.

John Balaguer, a graduate student now finishing his Ph.D. in materials engineering at RPI, reports: "The effort to simulate the process of welding continues at RPI as it did in the 1950s. We know a lot about welding certain kinds of materials, but every time a new material is developed, a whole set of actual or potential welding problems are developed along with it."

Balaguer cites as an example a project now underway to test the weldability of a very low carbon steel. "We are using the Gleeble to create a wide variety of microstructures using a small amount of material. Although we have only about 500 pounds of this new steel, the Gleeble allows us to do literally hundreds of tests," he says.

He adds, "Another attraction of the Gleeble is the capability to simulate mechanical transients — stress, load, strain — and to test whether thermal/mechanical effects could be synergistic. The Gleeble makes it easy to control force and strain, and it gives us a good handle on truly simulating a particular process."

Balaguer's interest is not just academic; he notes that, as materials become more complex, thermal/mechanical interactions begin to appear, and precise simulation becomes that much more important.

New processes are also a subject of study. "We currently have a proposal out to do some research on high energy density welding processes. At very high heating or cooling rates, the traditional rules of welding metallurgy can change a little, and we need to find out how."

Balaguer adds, "For me, among the benefits of working with the Gleeble are that it's functional, easy to use, and up-to-date. It is a dynamic system, always evolving in the direction of providing better information, broader capabilities, and a friendlier operator interface."

While Prof. Roger Wright agrees, the work performed under his direction using the Gleeble has taken a different turn — toward research involving hot workability, load relaxation, and sheet drawing.

"While most researchers regard the Gleeble as primarily a thermal test system, or perhaps a thermal-mechanical machine, we have found that, with 18,000 lb. capacity, the Gleeble is also a workhorse mechanical test system," Wright says.

He adds, "The new data acquisition system is among the best that I have seen. That and precise computer control make the Gleeble a good choice for performing painstaking work involving primarily mechanical processes."

For Bethlehem Steel and the Gleason Foundation, Wright and his students investigated the hot and warm workability of a number of materials, including SAE 1045 steel. Tension and load relaxation testing was performed on RPI's Gleeble 1500 to evaluate the strength and ductility of materials under strain rate and temperature conditions designed to simulate metal working.

"The Gleeble is well suited to achieving high precision results in this kind of research," Wright says.

IBM sponsored research under Wright's direction in the field of load relaxation. "It is actually a matter of major concern in mechanical connections for mainframe computers," Wright observes.

The crimped metal clips used to make some of the electrical connections within a computer can begin to relax after a while, presenting the potential of intermittent connections. This problem can be largely overcome by gold-plating the clips. Since this can be an expensive solution to the problem, IBM wanted to know more about load relaxation so that an alternative might be found.

Typical load relaxation test procedures on the Gleeble involved heating a specimen to a specified temperature. Next, the specimen was strained to a specific elongation. The crosshead was then stopped, and the specimen was held at temperature while the load was monitored.

The Gleeble also has found limited but effective use at RPI as a purely mechanical testing machine in a sheet drawing simulation project for General Motors.

Wright reports the test procedure involves pulling a strip of steel through a set of dies while measuring the stroke, the pulling load, the pressure holding the tools together, and the back force.

"Here we make full use of the Gleeble's relatively high crosshead speeds and data acquisition system, and the results are pointing the way to better ways of forming sheet metal. In the future, we may want to make some use of the Gleeble's heating system in this research," Wright says.

Wright adds, "Researchers are beginning to realize that, in addition to its thermal and thermal/mechanical capabilities, the Gleeble offers an additional degree of versatility as a highly competent mechanical test system. That is a rewarding discovery for any organization seeking to maximize the return on the dollars it spends on test systems."

This article first appeared in the Gleeble® Newsletter — Spring 1988.

 Breaking the Ice... The Gleeble at CANMET

Canadian icebreakers someday may be better able to withstand the ravages of smashing through the arctic ice pack, thanks in part to the Gleeble. Farfetched? Not at all.

At CANMET, a prestigious Canadian government research and technology center, research involving the Gleeble is underway that could have an impact on Canadian railways, steel mills, and marine vessels.

Dr. John Bowker, a research scientist at CANMET, is the leader of a project on continuous casting within the Steel Technology section of the Metals Technology Laboratories. He and his research group are involved in a cooperative program with Algoma Steel Corporation and Stelco (another steel company) to simulate continuous casting.

Using the Gleeble, they melt a small portion of a 10 mm round specimen and allow it to cool at a rate typical of a steel slab as it emerges from a continuous caster. The objective is to identify the "ductility trough" — the portion of the cooling cycle in which cracking is likely to occur as the hot slab bends from vertical to horizontal orientation as it leaves the caster.

"If the ductility trough is wide," Bowker says, "the probability is that the slab will crack and will therefore require scarfing to remove the cracked material." The continuous casting research project aims at improving the productivity of Algoma and Stelco's continuous casters by examining ductility troughs, developing cooling cycles that will reduce cracking, and exploring the effect of minor elements, such as niobium, aluminum, vanadium, and titanium, on cracking.

Bowker adds, "So far, we're finding good agreement between the results produced on the Gleeble and those observed at an operating continuous caster."

In another project, Bowker is using the Gleeble to simulate the thermal cycling that takes place when pieces of steel rail are flash butt-welded to form the longer sections that are placed on the road bed. The problem is that the welding process produces changes in the crystalline structure of the steel that sometimes result in soft spots. Since they wear unevenly and can produce rail buckling, the soft spots are a headache requiring additional rail maintenance.

Working with Algoma Steel, the Canadian National Railway, and Canadian Pacific Railway, CANMET personnel are using the Gleeble to carefully simulate the heat-affected zone in the welded rail. Their aim is to discover what processing conditions produce the troublesome soft spots and to develop welding procedures that will produce longer lengths of rail that are more uniformly hard.

At the same time, Bowker and others at CANMET are attacking the thorny and important issue of materials for naval, submarine, and icebreaking vessels. Researchers are examining the properties of HSLA steels as a replacement for HY steels.

Using the Gleeble, samples of the steel are heated and cooled, without loading, to simulate the single and double thermal cycles that occur when the steel plates for a marine vessel are welded together. Bowker says, "By performing these physical simulations, we can generate microstructures like those in the actual welds of marine vessels to see how heat input relates to the strength and toughness of the welded steel."

Bowker adds, "Toughness is a special concern because dynamic impact at low temperatures is part of an icebreaker's mission. If a crack occurs, it can lead to catastrophic failure.

"Our simulation work takes time because the microstructure in the heat-affected zone is inhomogeneous. As a result, we have to generate a small section of it at a time, test it, and then compare it with tests of a similar section from an actual weld. We're getting good agreement between the two," Bowker said.

In addition to thermal cycle simulation, considerable dilatometry work is being done at CANMET on steels for marine vessels. "We use the C-strain on the Gleeble to make careful measurements of the diameter of a specimen as it cools, and from that we are able to obtain information on the transformation behavior of the steel. This gives us another indication of the relationship between microstructure and toughness," Bowker said.

The Gleeble is also having an impact in an area that Bowker feels is often overlooked: weld corrosion. "With the Gleeble we're generating the exact microstructures we want and then testing them for corrosion. This is of considerable interest because icebreakers not only have to withstand cold and severe pounding, they have to do it in salt water," Bowker points out.

And what's in the Gleeble's future at CANMET? Bowker says, "We recently extended the capabilities of our Gleeble by adding the data acquisition system, the high speed load cell, and a larger vacuum tank. We'll use the Gleeble for a wide number of new projects, including analyzing the hot deformation behavior of micro-alloy steel at high strain rates."

At CANMET, the Gleeble will be breaking new ice in metallurgical research for a long time.

This article first appeared in the Gleeble® Newsletter — Summer 1988.

 Defining Character: The Gleeble at Alcoa

Richard P. Martukanitz, staff engineer, and his associates at the ALCOA Technical Center in Pennsylvania, are faced with a formidable challenge: to characterize the new advanced materials being developed by ALCOA before they are introduced to the people who will use them. Martukanitz says, "Our customers want to know how a material will behave when they are joining, forming, finishing, or machining it. It's our job to give them accurate answers relating to product manufacturing technology."

Fortunately, he and his colleagues have a formidable ally in their efforts to define the character of new materials: a Gleeble 1500 equipped with a vacuum chamber, an optional load transducer, and a data acquisition system. Installed three years ago, the Gleeble has become an integral part of materials characterization efforts at the ALCOA Technical Center. Martukanitz credits the high level of technical support provided by Dynamic Systems as a contributing factor in the success of the Gleeble at ALCOA.

Recently, the ALCOA team used the Gleeble to define the elevated temperature properties of aluminum-iron-cerium alloys. Developed for their light weight and strength at 700-800°F, these powder metallurgy alloys may find use in the National Aerospace Plane (NASP) and ordnance applications (some of the work was done at the request of an Israeli defense contractor). The question: what is the elevated temperature strength of these materials after rapid heating?

Using the Gleeble, samples were heated to 600, 700, and 800°F in 3-5 seconds, held at the target temperature for 5 seconds, and then tested mechanically. With this test regimen, the ALCOA team was able to accurately measure the tensile strength and yield strength of the aluminum-iron-cerium alloys.

Another project involved using the Gleeble to measure the crack sensitivity of certain aluminum alloys during casting. "Our interest in the answer was more than technical," Martukanitz says, "because crack sensitivity affects the pricing of the material."

The objective was to quantitatively differentiate crack sensitivity among a group of alloys. The testing routine involved heating samples on the Gleeble to their nil strength temperatures (between 1100 and 1200°F), reducing the temperature in 100 increments, and then pulling the samples until failure.

By measuring the necked area of each sample and plotting it against the temperature at which failure was induced, the ALCOA researchers were able to build up a profile of temperature vs. ductility for each alloy.

An analysis of these results revealed that the shape of the ductility profile can vary widely, depending upon the constituents of the alloys. As a result of this testing program, ALCOA now has a better handle on how to reduce crack sensitivity during casting through control of alloy composition and processing temperatures.

The Gleeble was also invaluable for research at the ALCOA Technical Center involving diffusion bonding of titanium-aluminide materials and forming of aluminum-lithium alloys.

Titanium aluminide is a new material that may find application in advanced aircraft components. Diffusion bonding will be used to join assemblies, and the team at ALCOA built a special set of jaws designed to permit diffusion bonding of lapped samples on the Gleeble. The Gleeble is then used for testing the sheer strength of the resultant bond.

"The Gleeble was ideal for this work because of its highly accurate control and measurement of temperature, time, and force," Martukanitz says, adding: "The Gleeble is a very powerful machine, and it takes a good operator and imagination to use it to its utmost."

The work with ALCOA's aluminum-lithium alloy 2090 had an esthetic twist to it. Commercial aircraft manufacturers found that the process of forming aluminum-lithium sheets to create the skin of the aircraft occasionally produced Luder's bands, or "Ludering" — a kind of metallurgical equivalent of stretch marks that appears at intervals in the metal. While Ludering is structurally insignificant, it can be an unsightly nuisance, particularly since, to save weight, many aircraft are no longer painted. The aircraft builders asked Martukanitz and his colleagues to find a way to prevent the bands from occurring.

Using the Gleeble, the ALCOA researchers stretched samples of the alloy and accurately plotted load and strain vs. time. The data they obtained was used to alter the fabricating procedures and change the processing temperatures. As a result, the occurrence of Luder's bands during forming of 2090 has been greatly reduced.

Martukanitz says, "Having the Gleeble has allowed us to expand the scope our work, and the Gleeble makes it easier to obtain results with greater accuracy and precision. Right now, it looks like we'll be keeping the Gleeble busy."

This article first appeared in the Gleeble® Newsletter — Fall 1988.

The Ohio State University 

At Ohio State, the only university in the US that offers a doctorate in Welding Engineering, you'll find a Gleeble 1500 system in almost constant use. And for good reason: a nearly endless procession of graduate students and faculty members are conducting a variety of research projects aimed at improving their knowledge of how to effectively and economically join materials.

Dr. William A. "Bud" Baeslack, Professor in Ohio State's Depart-ment of Welding Engineering, says, "Most of the work involves joining advanced materials — such as non-ferrous alloys, stainless steels, nickel-based alloys, aluminum-lithium, and titanium — for a variety of applications, many of them in the aerospace field."

"We are constantly getting new problems from a joining standpoint," he says. "The projects are diverse because of the variety of the materials and because materials keep changing."

For General Electric Aircraft, Dr. Baeslack's colleagues are using the Gleeble to look at the weldability of cast Inconel 718 superalloys for turbine engines. They're examining the effect of alloy chemistry on cracking in the weld heat affected zone.

In a related project for the Edison Welding Institute, the Gleeble itself is being studied as a predictor of the weldability of 900 series superalloys. These superalloys exhibit very low expansion which makes them well suited for the high temperature environment inside a jet engine. Several different heats of the superalloys are tested using the Gleeble and then evaluated again using other weldability testing techniques. The results are then compared to see how well they correlate. According to Dr. Baeslack, preliminary results show "moderately good correlation."

Ohio State researchers are also performing continuous cooling phase transformation studies of advanced titanium aluminide alloys that may someday find application in the National AeroSpace Plane (NASP). Through careful control of the cooling rate, coupled with dilatometer measurements, they are able to simulate the HAZ structure of these materials so that structure and property analysis can be performed.

Another project, supported by the Office of Naval Research, involves simulating fusion zone specimens of aluminum-lithium alloys to see what effect different weld solidification rates have on the microstructure. A small quartz tube is clamped in the Gleeble's jaws, with a sample of the alloy inside. The sample is melted and then allowed to cool at a controlled rate. Dr. Baeslack reports that the structure of the resultant specimens closely parallels the structure found in samples produced through actual welding processes.

The US Army is supporting a fundamental research project at Ohio State involving diffusion bonding of advanced aluminum alloys for high temperature applications. Using the Gleeble, diffusion bonds with different characteristics are generated under precise temperature and pressure control. The quality, strength, and integrity of the bonds are then evaluated through a variety of methods, and the results are plotted against the bonding parameters. Ultimately this work could have possible application in missiles.

In the area of weld simulation with more conventional materials, Ohio State researchers are looking at duplex stainless steels — 50% ferrite, 50% austenite — to examine the effects of the thermal cycle on toughness, microstructure, and corrosion of the HAZ. By changing the peak temperature and cooling rate, they can determine which parameters produce welds that are well suited for applications in corrosive environments.

"The Gleeble's ability to simulate precisely the thermal and mechanical conditions present during welding is a key to this work," Dr. Baeslack says.

Dr. Baeslack adds, "Nearly every one of my students makes use of the Gleeble at some point in his or her research. It's an essential tool for what we are doing because it can control temperature, heating rates, and cooling rates, all very precisely. The Gleeble is also unique in its flexibility — it allows us to do a lot with just one piece of equipment."

This article first appeared in the Gleeble® Newsletter — Winter 1988/89.