Preheat and Post Weld Heat Treatment
Chrome-moly is air hardening. Everything about welding and fabricating it follows from that one fact.
The metallurgy that makes chrome-moly good at temperature also makes it difficult to weld. It hardens on cooling in still air, with no quench required, and the hard material lands right beside a weld that contains dissolved hydrogen and residual stress. That combination cracks, and it cracks on a delay.
What goes wrong
Welding heats a band of parent metal beside the joint above its transformation temperature. As the arc moves away, that band cools rapidly, quenched by the surrounding cold mass of the flange. In an air hardening steel that produces martensite: hard, strong and brittle.
Three things are now present in the same place:
- Hard brittle martensite in the heat affected zone, with very little ductility to accommodate strain.
- Dissolved atomic hydrogen, introduced by the welding process from moisture in flux, coatings, surface contamination or humidity.
- High residual stress, locked in by the weld shrinking against a rigid section.
Hydrogen migrates to the regions of highest stress in the hardest material and initiates cracking. The process takes time, which is why it is called delayed or hydrogen induced cracking. A joint can look perfect, pass inspection and hydrotest, and crack weeks later.
This is why a skipped PWHT is not caught by testing. The joint holds pressure on day one. Nothing in a hydrotest interrogates the toughness of the heat affected zone. The evidence that the heat treatment happened correctly is the heat treatment chart and the hardness result, not the pressure test.
The controls
Preheat
Heating the joint area before and during welding slows the cooling rate through the transformation range, so less martensite forms and what does form is tempered by subsequent passes. It also evaporates surface moisture, removing a hydrogen source.
Required preheat rises with chromium content and with section thickness. Typical practice runs from a few hundred degrees F on F11 and F12 up to appreciably higher on F91, and the governing numbers come from the design code and the welding procedure specification, not from habit.
Preheat has to be maintained, not just established. Checking it once with a crayon at the start of the shift and then letting it drift is a common and consequential shortcut.
Interpass temperature
The minimum interpass temperature keeps the joint inside the safe window throughout welding. There is usually a maximum as well, because excessive heat input coarsens the grain and degrades toughness. Both limits belong on the procedure.
Low hydrogen practice
- Low hydrogen consumables, correctly designated
- Electrodes and flux stored in heated ovens and issued in quivers, not left on the bench
- Joint preparation clean, free of oil, paint, rust and moisture
- Shielding gas dry, with attention to humidity in the shop
Hydrogen is put into the weld by the fabricator. Every one of these measures is about not putting it there in the first place, which is easier than removing it afterwards.
Post weld heat treatment
PWHT tempers the martensite back to a tough structure and relieves residual stress. It also allows remaining hydrogen to diffuse out. For chrome-moly above F1 it is effectively mandatory, with the design code setting temperature and hold time by grade and thickness.
Heating and cooling rates are controlled too, and thermocouples have to be placed so they record what the joint actually experienced rather than what the furnace was set to. The chart is a deliverable.
Maintaining preheat until PWHT
On heavy wall and higher chromium work, the joint should not be allowed to cool to ambient between welding and PWHT. Either maintain preheat through to the heat treatment, or perform an intermediate dehydrogenation heat treatment, commonly called a hydrogen bake-out, which drives hydrogen out so the joint can cool safely and wait.
Hardness verification
Hardness after PWHT is the measurement that proves the treatment achieved its purpose. Limits depend on grade, code and service, with many specifications working in the region of 225 HBW for general chrome-moly service. Sour service under NACE MR0175 and ISO 15156 is tighter, commonly 22 HRC.
Too hard means under-tempered, with the brittleness still present. Too soft can mean over-tempered, which on a creep service grade means the creep strength the design relied on is no longer there. Both directions are failures.
Grade 91 is a special case
F91 has the best creep strength in the chrome-moly family and the narrowest processing window. Its properties come from a specific normalized and tempered condition, and they are fragile in a way that F11 and F22 properties are not.
The trap: PWHT above the lower critical temperature. Grade 91 tempering runs close to the temperature at which the steel begins to re-austenitize. Overshoot it and the material transforms, then forms fresh untempered martensite on cooling. The engineered condition is destroyed. Another temper will not bring it back. Recovery means a full re-normalize and temper, which on an installed component is usually not practical.
Which explains two things that otherwise look like fussiness:
- The nickel plus manganese restriction. Both elements lower the lower critical temperature. A heat running high on them narrows or closes the window between an effective temper and accidental re-austenitization. The restriction keeps a usable gap.
- The insistence on calibrated control and recorded charts. With a window this narrow, "about right" is not a defensible position.
Grade 91 also has a long term failure mode worth knowing about. Under sustained creep loading, cracking can develop in the fine grained region of the heat affected zone, a mechanism usually referred to as Type IV cracking. It appears after long service rather than during fabrication, and it is one reason weld locations and inspection planning on Grade 91 systems get the attention they do.
More in Grade 91 flanges and why heat treatment decides everything.
Typical requirements by grade
| Grade | Preheat | PWHT | Sensitivity |
|---|---|---|---|
F1 | Modest | Often required by thickness | Low |
F12, F11 | Moderate | Required | Moderate |
F22, F21 | Higher | Required | Higher. Heavy wall work adds temper embrittlement chemistry restrictions |
F5, F9 | Higher | Required | High. Strongly air hardening |
F91 | Highest | Required, within a narrow window | Highest. Overheating destroys the condition irreversibly |
What this means when you order
A flange arrives from us in a certified heat treatment condition, and the fabricator then welds it into a system and applies PWHT to that joint. Both halves have to line up, so:
- State the grade and the forging class. The class is the heat treatment condition, and it has to match what the design calculation assumed.
- Tell us the hardness limit and whether NACE MR0175 or ISO 15156 applies.
- Tell us about restricted chemistry if the service imposes J-factor or X-bar limits, because that decides which heats we can offer at all.
- Tell us about impact testing and the test temperature.
- Say if you need the heat treatment chart and hardness results in the documentation package.
And confirm your fabricator has a qualified procedure for the actual grade, class and thickness. A procedure qualified on F11 does not cover F91, and on this material family that distinction is the difference between a sound joint and a delayed failure.
See chrome-moly grades for the material reference and high temperature and hydrogen service for the damage mechanisms behind the grade choice.
Common questions
Why does chrome-moly need preheat when carbon steel often does not?
Because chrome-moly is air hardening. It transforms to hard brittle martensite as it cools from welding temperature in still air, with no quench required. Preheat slows the cooling rate through the transformation range so less martensite forms and what does form is less severe, and it drives off surface moisture that would otherwise contribute hydrogen.
Can I skip PWHT if the weld passes hydrotest?
No, and hydrotest is exactly why this is dangerous. An untempered heat affected zone holds pressure perfectly well on day one. The failure mode is delayed cracking, appearing days to months later as hydrogen and residual stress do their work on hard brittle material. No pressure test detects it. The design code sets the PWHT requirement and it is not a quality preference.
What is the danger of overheating a Grade 91 PWHT?
Exceeding the lower critical temperature re-austenitizes the material. On cooling it forms fresh untempered martensite, and the carefully engineered normalized and tempered condition that gives Grade 91 its creep strength is gone. Another temper will not restore it. Recovery requires a full re-normalize and temper, which on an installed component is usually impractical.
Why are nickel and manganese restricted in Grade 91?
Because they lower the lower critical temperature. Grade 91 PWHT already runs close to that limit, so a heat with high nickel plus manganese narrows or eliminates the window between an effective temper and accidental re-austenitization. Restricting the sum keeps a usable gap. It is a real restriction, not a formality.
What hardness should a chrome-moly weld be after PWHT?
It depends on the grade, the code and the service. Many specifications work to a limit in the region of 225 HBW for general chrome-moly service, and sour service under NACE MR0175 and ISO 15156 is tighter, commonly 22 HRC. The specific limit comes from the governing specification. What matters is that hardness is measured and recorded, because it is the evidence the heat treatment worked.
Can a weld be left overnight before PWHT?
On thin sections and lower grades sometimes, but on heavy wall and higher chromium grades letting a joint cool to ambient before PWHT is how delayed cracking happens. Normal practice is to maintain preheat until PWHT, or to perform an intermediate dehydrogenation heat treatment, a hydrogen bake-out, so the joint can be allowed to cool safely.
Chrome-moly flanges with the documentation to match
Grade, forging class, hardness limit and any chemistry restriction. We will supply the material and the paperwork that proves it.
Send the print, or send the size, class and grade. Texas Flange quotes quickly.
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