Chrome-moly does not forgive welding shortcuts, and the punishment arrives late. A joint welded without proper preheat and post weld heat treatment will usually pass visual inspection, pass radiography and pass hydrotest. Then it cracks, weeks or months into service. Understanding why makes the requirements obvious rather than bureaucratic.
Air hardening
Carbon steel needs a quench to form martensite. Chrome-moly does not. The alloying that gives it high temperature strength also shifts the transformation behaviour so that cooling in still air is enough. That is what air hardening means, and it is the root of everything that follows.
When you weld, a band of parent metal beside the joint is heated above its transformation temperature. The arc moves on, and that band is quenched by the surrounding cold mass of the flange, which on a heavy Class 900 weld neck is a very effective heat sink. The result is a narrow zone of hard, brittle martensite immediately beside the weld.
Three ingredients, one crack
The heat affected zone now contains everything delayed cracking needs.
- Hard brittle material with very little ductility to accommodate strain.
- Dissolved atomic hydrogen, introduced by the welding process itself from moisture in flux and coatings, surface contamination, or humidity in the shop.
- High residual stress, locked in as the weld metal shrinks against a rigid section.
Hydrogen diffuses to the regions of highest stress in the hardest material, and initiates cracking there. It takes time, because diffusion takes time. That delay is the whole problem: the inspection happens before the crack does.
No pressure test detects an untempered heat affected zone. Hydrotest demonstrates that the joint holds pressure today. It says nothing about the toughness of a narrow band of martensite beside the weld. The evidence that heat treatment happened correctly is the chart and the hardness result.
Preheat
Preheat does two things. It slows the cooling rate through the transformation range, so less martensite forms and what does form gets tempered by subsequent weld passes. And it evaporates surface moisture, removing a hydrogen source before it can get into the weld.
Required preheat rises with chromium content and with section thickness. F11 and F12 need a moderate preheat, F22 more, F5 and F9 more again, F91 the most. The governing numbers come from the design code, ASME B31.1 or B31.3 as applicable, and from the qualified welding procedure. They are not a matter of shop preference.
The common failure is treating preheat as an event rather than a condition. Checking it once with a temperature crayon at the start and then letting it drift as the shift goes on defeats the purpose. It has to be maintained and monitored across the whole joint, including the far side of a thick section.
Interpass temperature
The minimum interpass temperature keeps the joint inside the safe window for the duration of welding. There is usually a maximum as well, because excessive accumulated heat coarsens grain structure and degrades toughness.
Both limits belong on the procedure, and both get checked. A procedure that specifies only a minimum is incomplete for this material.
Low hydrogen practice
All the hydrogen in the weld was put there by the fabricator. These measures are about not putting it there.
- Low hydrogen consumables, correctly designated for the grade
- Electrodes and flux stored in heated ovens, issued in heated quivers, and not left out on the bench between passes
- Joint preparation clean and bright, free of oil, paint, rust, scale and moisture
- Dry shielding gas, with attention paid to shop humidity
- No moisture on the part, including condensation on cold metal brought in from outside
Post weld heat treatment
PWHT does three jobs at once:
- Tempers the martensite back to a tough structure
- Relieves residual stress, removing one of the three ingredients
- Lets remaining hydrogen diffuse out, removing another
For chrome-moly above F1 it is effectively mandatory, with the code setting temperature and hold time by grade and thickness. Heating and cooling rates are controlled too, and thermocouples have to be placed so the chart records what the joint experienced, not what the furnace controller was told. That chart is a deliverable, not an internal record.
Do not let it cool and wait
On heavy wall and higher chromium grades, letting a completed joint cool to ambient and sit overnight before PWHT is how delayed cracking happens. Standard practice is one of two things:
- Maintain preheat continuously from welding through to PWHT, or
- Perform an intermediate dehydrogenation heat treatment, a hydrogen bake-out, which drives hydrogen out at a lower temperature so the joint can then cool safely and be heat treated later.
Hardness verification
Hardness after PWHT is the measurement that proves the treatment did what was intended. Limits depend on grade, code and service. Many specifications work in the region of 225 HBW for general chrome-moly service. Sour service under NACE MR0175 and ISO 15156 is tighter, commonly 22 HRC.
Read it in both directions:
- Too hard: under-tempered. The brittleness is still there.
- Too soft: over-tempered. On a creep service grade, the strength the design relied on may be gone.
Grade 91 needs its own plan
F91 tempering runs close to the temperature at which the steel re-austenitizes. Overshoot and the engineered condition is destroyed irreversibly, and another temper will not restore it. That is why nickel and manganese are restricted, and why calibrated control and recorded charts are non-negotiable on Grade 91 work.
Detail in Grade 91 flanges and why heat treatment decides everything.
Practical checklist
- Welding procedure qualified for the actual grade, forging class and thickness
- Preheat established across the full section and maintained, not spot checked once
- Interpass minimum and maximum both specified and monitored
- Low hydrogen consumables from a heated oven, in quivers
- Preparation clean, bright and dry
- Preheat maintained to PWHT, or a dehydrogenation treatment performed
- PWHT to code temperature and hold, with controlled ramp rates
- Thermocouples placed on the joint, chart recorded and retained
- Hardness tested against both limits and recorded
Full detail on preheat and post weld heat treatment.