Test Methods for the High-Temperature Strength of Molybdenum Electrodes

In electrode design, every decision about span, cross-section and current density ultimately answers one question: how much strength the material has left at 1,300 degrees. The answer comes not from room-temperature values in a handbook but from targeted testing. Molybdenum electrode strength data must be based on the hot state, and this point cannot be overstated. Test data are the more expensive input in molybdenum electrode design formulas, yet far cheaper than a single broken-electrode incident.

High-Temperature Tensile Testing: The Strength Baseline

A standard specimen is pulled in a heating furnace under vacuum or high-purity argon, with temperature coverage from room temperature to 1400 °C, producing a curve of strength against temperature. Three points matter: the atmosphere must be oxygen-free, otherwise the specimen oxidizes and loses weight during heat-up and the data read low; the gripping section must be water-cooled and isolated so that fracture occurs in the isothermal zone; heating and loading rates must be standardized to ensure comparability between batches; reduction of area should be recorded at the same time, because it reflects the tendency to hot brittleness and exposes material anomalies and process drift earlier than strength figures do. This curve is the direct basis for checking suspension stress and allowable load, and the first input for molybdenum electrode span design.

Creep and Stress Rupture: Long-Term Behavior

Tensile testing gives an instantaneous value, while service demands a time integral. The constant-load creep test records the strain rate and steady-state stage at high temperature and answers how fast a molybdenum electrode sags; the stress-rupture test measures time to fracture at constant temperature and load, usually taking the 100-hour rupture stress as the design index, and long-term extrapolation must be handled conservatively. The test cycle is long and costly, so it is normally performed by sampling per grade and batch rather than on every single electrode. For dispersion-strengthened grades such as TZM and lanthanum-doped molybdenum, these tests best demonstrate the life gain over pure molybdenum, and they are the touchstone that distinguishes one grade from another.

Bend Testing: Ductile-to-Brittle Transition

• Small three-point bend specimens are tested over a series of temperatures, and the ductile-to-brittle transition temperature is defined by the shift in fracture morphology from brittle to ductile;
• This index sets the safety boundary for installation, maintenance and cold furnace operation, and it is the basis for the minimum operating temperature during winter installation and maintenance;
• Recrystallization state, grain size and oxygen content all shift the transition temperature, which makes it a sensitive probe for structural sensitivity.

How to Use the Data

Combining the three sets of data gives electrode design a complete chain: tensile data set the allowable stress, creep data set the span and the replacement interval, and the transition temperature sets the operating rules. WeiBeam insists on measuring new material grades rather than applying literature values, because differences in powder metallurgy and forging practice are enough to shift a curve by more than 100 MPa. Before a new furnace is commissioned, we recommend a confidence check on retained specimens, so that design assumptions are pinned to your own material rather than copied from literature data for the grade.