Hot Tensile Testing - Improving Efficiency
Meeting Nadcap high-temperature testing requirements
This article was originally published on April 13, 2014.
When hot tensile testing is required, it can be time consuming and energy intensive, especially when trying to meet strict standards such as Nadcap. With exacting temperature-uniformity range requirements of the furnace and test specimen, it can take a few hours to reach and stabilize to the required temperatures and only minutes to complete the test. In today’s fast-paced manufacturing environment, waiting for specimens to be tested can cause unwanted delays in product shipment and potentially affect quarterly and annual profits.
If materials need to perform at elevated temperatures, suppliers are often required to certify their parts. The high-temperature mechanical properties of a material can be determined through hot tensile testing. There are multiple testing configurations and heating techniques that vary based on the application. In general, the equipment required for this type of testing is a mechanical loading system to apply force to a specimen and equipment to heat it. The system also includes components to control, measure and collect data.
This article will discuss a common test configuration. The equipment required to perform this testing is a universal test machine (UTM), a furnace and an extensometer (Fig. 1). The UTM should be equipped with specimen couplings, pull rods and quick-change adapters. The system should be equipped with calibrated thermocouples and instruments to measure, control and record temperatures from the furnace and the specimen. The selection of extensometry depends on the application requirements.
Tensile-Test Machine Frame
The UTM frame selection will be based on the testing specification being followed. There are two basic types of UTMs available on the market: an electromechanical or a servo-hydraulic model. Each has distinct advantages and should be explored with a knowledgeable application technician. When evaluating the addition of this test to a lab’s capabilities, it is important to note that it is sometimes possible to retrofit an existing UTM frame to add the capability of hot tensile testing.
Specimen couplings, pull rods and quick-change adapters must be made of material that can withstand the elevated temperatures inside the furnace.
A common furnace used in hot tensile testing is a multiple-zone, ATS model 3210 split-tube furnace (Fig. 2). The multiple-zone configuration allows for tighter tolerance control. The split construction allows for faster loading and unloading of the specimen. Standard furnace construction features include low k-factor rigid insulation, a stainless steel shell and end flanges, and replaceable heating elements.
The test temperatures in the furnace and of the specimen must be accurately controlled due to the typically strong temperature dependence of tensile ductility. For this reason, three calibrated thermocouples are attached to the surface of the specimen using thermocouple quick tips, welded or twisted (Fig. 3). This ensures precise temperature control and temperature uniformity on the specimen.
An extensometer is used to measure changes in the length of the specimen. There are several types of extensometers available, and the selection of extensometry is dependent on the testing requirements. Available extensometers are either side-mounted, bottom-mounted or laser types. The preferred method is to use a rod-and-tube type frame, like the ATS model 4112-T. With a frame, only one extensometer clip is needed, and it is attached once the furnace is in place for testing. This allows the operator to set up multiple furnaces and specimens for preheat while eliminating any movement of the extensometer on the specimen during the placement of the furnace in the test frame.
Setting up the hot tensile test is simple and can be completed in just minutes. The operator running the test ensures the appropriate couplings, pull rods and adapters are set up on the UTM. The samples (with thermocouples in place) are then loaded into the furnace as instructed in the applicable test procedure. The furnace is closed and the heating process begins, with all temperatures being controlled and recorded by a computer program such as the ATS WinHT.
WinHT has precise temperature control, which keeps testing temperatures within the strict Nadcap and ASTM requirements. WinHT has a pre-emptive power compensation, which automatically eliminates temperature changes due to line-voltage fluctuations. It will create reports and graphs for each sample.
Once the specimen is ready, it is loaded into the test frame, the extensometer is put in place and the test is performed. Within minutes, the test is over and the data is recorded. This data can be used by the operator to determine tensile strength, yield strength, ductility and ultimate strength.
The test specification and production methods of the specimen material dictate the heating rate. It is common to find a low heating-rate requirement with an additional “soak” time of 10-60 minutes prior to the application of the load. With such lengthy wait times for the furnace and specimen to reach the required temperatures, it is difficult to test more than several specimens in any given standard work day.
In order to increase productivity in the test lab, an ATS carousel system along with WinHT can be used in place of a single furnace. A typical carousel will have three, four or six furnaces that pivot from a center support and can be easily loaded on the test frame. If using the six-furnace option, for example, the operator can set up six tests each using two pull rods, specimen couplings and extensometry (measures the amount of displacement the specimen makes while being tested) to mount both ends of the specimen to (Fig. 4).
The operator then heats the furnaces to the required temperature, waits for the required soak time and mounts the entire assembly into the furnace using the quick-change couplings. The specimen is then tested to failure, the extensometers measure the displacement of the specimen, and the furnace controller measures the temperature at failure. The computer program records this information as well as the time to failure and other information as required in ASTM E21.
Once the test is completed, the operator removes the specimen and rotates the entire carousel assembly to insert the next specimen in place. There are now six tests completed back to back and six furnaces preheated and ready to start heating up the next set of specimens without losing precious time and furnace temperature.
By increasing the number of samples that can be tested in a day, the efficiency of the lab is increased. Whether the lab is part of a primary-metals plant or an independent test lab, production capacity increases and profits should too.
In conclusion, everyone from the metal producer to the end customer benefits from the improved testing efficiency a carousel system provides. IH
For more information: Contact Robert Antolik, sales engineer, Applied Test Systems, 154 East Brook Lane, Butler, PA 16002; tel: 724-283-1212; fax: 724-283-6570; e-mail: firstname.lastname@example.org; web: www.atspa.com