What is Hot Hardness Testing?
By Engaged Expert Dr. John Tartaglia
Hot hardness testing is the measurement of material hardness while at elevated temperature. Hot hardness can be valuable for assessing and comparing materials, such as tool steels and wear resistant coatings, which are used in high temperature applications such as valve seats and dies.
Because room temperature hardness changes relatively little after high temperature exposure, it cannot be used to assess high-temperature properties. However, hardness-at-temperature changes significantly and reflects high temperature properties such as resistance to high temperature wear, erosion, indentation, plastic deformation, tempering, aging and creep.
Hot hardness can characterize a material more quickly and inexpensively than hot tensile, prototype, or service testing because multiple temperatures and exposure times can be tested in a single hot hardness testing run. This makes hot hardness testing an excellent way to compare multiple materials at elevated temperature.
How does Element conduct hot hardness testing?
Element is the only commercial laboratory that has hot hardness testing capability. Element conducts hot hardness testing similarly to normal Vickers hardness testing, except that the test is conducted inside a vacuum chamber at an elevated temperature.
Samples measuring 25 by 9.5 by 9.5 mm (1 inch by 3/8 inch by 3/8 inch) are metallographically polished to a 1 μm finish to prepare them for testing. As many as three samples are inserted into the hot hardness test chamber and evacuated to 10-6 torr to prevent sample oxidation at high temperatures.
Resistance elements heat the chamber while thermocouples monitor the temperature of each sample. Each test temperature is stabilized for at least 5 minutes prior to making Vickers hardness impressions, although exposure times up to multiple hours are quite possible. Three Vickers impressions are made on each sample using a 10 kg load, then the chamber is heated to the next higher temperature in the series and the impression sequence is repeated.
Element can conduct Vickers macrohardness testing at room temperature and at elevated temperatures from 175 to 870°C (350 to 1600°F); as many as 8 temperatures can be tested on each sample. After the samples have cooled, an Element expert removes them from the chamber, measures the diagonal lengths, and computes Vickers macrohardness using a formula from the ASTM E92 test method.
A typical tested sample is shown in Figure 1 and typical hot hardness data are shown in Figure 2.