Temper Embrittlement: The 375–575 °C Trap in Cr-Mo Steels

The phenomenon

Temper embrittlement is a loss of impact toughness that develops when certain alloy steels are held in, or cooled slowly through, approximately the 375–575 °C range. The steel's tensile properties and hardness barely change — a part can pass every routine test and still be brittle at room temperature, with the ductile-to-brittle transition temperature shifted sharply upward. It is best known in chromium and Cr-Mo steels, including the 2.25Cr-1Mo and Cr-Mo-V families used for pressure vessels, rotors and heavy bolts.

The mechanism: impurities at grain boundaries

The embrittlement is not caused by the main alloying elements themselves. Trace impurities — chiefly phosphorus, tin, antimony and arsenic — segregate to prior-austenite grain boundaries while the steel sits in the critical range, weakening the boundaries so fracture runs between the grains. Manganese and chromium promote the segregation; molybdenum in moderate amounts retards it, which is one reason Mo appears in so many pressure-vessel grades.

Two features follow from the mechanism:

• It is reversible. Reheating above the range (and cooling reasonably fast) redissolves the segregation and restores toughness — hence "reversible temper embrittlement", distinct from the one-way embrittlement of tempered martensite at lower temperatures.
• It is time-dependent. Short passes through the range do little harm; long holds, slow furnace cooling of heavy sections, and years of service inside the range accumulate damage.

How specifications fight it

Because the cause is chemistry, the defence is written into purchasing requirements:

• Impurity limits — refiners control P, Sn, Sb and As to low levels, and buyers of critical forgings specify combined embrittlement parameters (the widely used J-factor and X-bar formulas weight these elements together) in the purchase order.
• Tempering practice — temper above the range where possible and avoid slow cooling through it in heavy sections; step-cooling test cycles are used to qualify material for vessels that will live in the range.
• Acceptance by impact testing — Charpy transition-temperature requirements, sometimes after a simulated embrittling cycle, verify the actual toughness rather than assuming it.

Practical reading

If a Cr-Mo component has spent long service at 400–550 °C and must be assessed, hardness alone says nothing about its toughness. Conversely, a new forging that meets tight impurity limits and is tempered correctly is not "at risk" merely because its tempering temperature was chosen near the range — the phenomenon needs the impurities, the temperature and the time together.

How it is detected

Because hardness and tensile strength stay normal, temper embrittlement is found by Charpy impact testing — specifically by the shift in the ductile-to-brittle transition temperature compared with the un-embrittled condition. Toughness measured at a single temperature can mislead; the transition curve is the evidence. Where a component's history is suspect, samples trepanned from the part or prolongations of the original forging are tested, sometimes after a de-embrittling reheat to confirm the diagnosis: if toughness returns after the reheat and fast cool, temper embrittlement — rather than permanent damage such as creep or hydrogen attack — was the cause.