Why Preheat and Interpass Temperature Matter

A weld often gets rejected without showing any visible discontinuity, because the real problem does not sit in the weld itself. It hides underneath and beside it, in the heat-affected zone (HAZ). Preheat and interpass temperature rank among the most critical parameters that decide whether a joint stays reliable for its whole service life. At first glance both look like nothing more than "warming the metal up a bit". In reality they are thermal control tools that govern the cooling rate, the time hydrogen has to escape from the weld, the HAZ hardness, and the final toughness. Apply them wrong and you get delayed cold cracks that surface days or even weeks after erection. Apply them right and you get a joint that works without trouble for decades. This article covers the metallurgical logic, the calculation basis, and the field control of preheat and interpass temperature through the eyes of an inspector.

1. Cooling rate in welding: the invisible variable

A weld cools from melting temperature toward room temperature within seconds. How fast that happens is the single most basic factor deciding which microstructure forms in the HAZ. Engineers normally use the cooling time from 800 °C down to 500 °C (t8/5) as the reference, because most phase transformations in steel take place inside that band.

If cooling runs too fast (short t8/5), austenite has too little time to diffuse and transforms into hard, brittle martensite. That does not mean "slower is always better". Toughness does not rise monotonically with t8/5; there is an optimum window (typically on the order of 5–25 s in structural steels, with the exact range depending on the material and the yield strength class). A very short t8/5 produces hard martensite. A very long t8/5 produces grain-boundary ferrite, upper bainite, and M-A islands in the coarse-grained HAZ. Both extremes cut toughness, and at very long t8/5 toughness can even end up worse than in martensite. The primary function of preheat is to hit that window: raise the starting temperature of the weld area, slow the cooling rate, and stretch t8/5 under control.

Preheat is not the only lever on t8/5, though. Put simply, t8/5 = f(preheat temperature, heat input, thickness, and heat flow geometry). Heat input is defined in the WPS and can be measured directly, but skip the unit conversion and the formula will not give you kJ/mm:

Q = (U × I × k) / (v × 1000) — where v is in mm/s Q = (U × I × k × 60) / (v × 1000) — where v is in mm/min

Here Q is heat input (kJ/mm), U is arc voltage (V), I is welding current (A), v is travel speed, and k is the process-dependent thermal efficiency factor. EN 1011-1 and ISO/TR 18491 give these typical k values: SAW 1.0; SMAW, GMAW and FCAW 0.8; GTAW 0.6. One distinction matters a lot: heat input and arc energy are not the same thing. If you did not apply the k factor, the number in your hand is arc energy, and the WPS must state clearly which of the two it limits.

Whether the heat flow is two-dimensional (thin section) or three-dimensional (thick section) also shifts t8/5 noticeably. So never assess preheat on its own. Assess it together with the heat input range and the joint geometry. The calculation methods in EN 1011-2 use exactly those inputs together.

2. Hydrogen diffusion and why we need to buy time

During welding, hydrogen dissolves into the molten pool from moisture, oil, rust, or the electrode coating. This diffusible hydrogen (HD) stays mobile inside the metal while the temperature is high, and it can escape freely. The critical step in the mechanism is this: hydrogen solubility is high in austenite (FCC) and very low in ferrite and martensite (BCC). At the moment of transformation the structure suddenly becomes supersaturated. By the time the metal reaches room temperature, hydrogen sits trapped in the most critical regions of the HAZ, in traps and inside martensite.

The quantity that really explains the behavior is not solubility but the diffusion coefficient (diffusivity). Hydrogen diffusivity in BCC ferrite runs orders of magnitude higher than in austenite (typically 4–5 orders). That is exactly why preheat speeds up hydrogen escape: if the structure is still hot after transforming to BCC, hydrogen can diffuse out fast. The same principle explains why austenitic stainless weld metal resists hydrogen cracking so well. Hydrogen dissolves in the FCC structure, but its diffusivity is low, so it does not easily build up to the local saturation that starts a crack in critical regions.

Preheat and adequate interpass temperature extend the time the weld spends at high temperature, and that gives hydrogen its escape window. In other words, preheat controls not only the cooling rate but also the window during which hydrogen can diffuse out.

The same logic extends into dehydrogenation heat treatment (DHT / post-heating), applied immediately after welding ends. Typical practice sits around 200–300 °C for 1–4 hours depending on thickness, and the common rule is 1 hour per 25 mm of thickness, minimum 2 hours. Miss either of these two conditions and the treatment loses most of its value:

The post-heating temperature must not be lower than the preheat temperature.
Never let the part drop below its preheat / interpass temperature before post-heating starts. DHT applied after the part has cooled does not prevent the crack; it only delays it. The hydrogen is already trapped, and the crack may already have started.

Do not confuse DHT with post-weld heat treatment (PWHT). Post-heating runs at a much lower temperature, serves a different purpose, and where PWHT applies, it comes before PWHT as a separate operation.

Cut hydrogen off at the source

Preheat is only one front in the fight against hydrogen. The measurable, auditable front is consumable management:

HD measurement: Determine diffusible hydrogen content to EN ISO 3690 and report it in ml/100 g.
Consumable classification: Consumable designation shows H5 / H10 / H15 on the EN ISO 2560 side (≤ 5, ≤ 10, ≤ 15 ml/100 g respectively), and H4 / H8 / H16 on AWS A5-series consumables.
Converting the class into the calculation: The HD value in the consumable designation is not a direct input to the calculation method. Convert it into the hydrogen scale of the code you are working to. EN 1011-2 Method B (CEV) takes hydrogen on an A–E scale (A: >15, B: 10–15, C: 5–10, D: 3–5, E: <3 ml/100 g). EN 1011-2 Method A (CET) takes HD directly in ml/100 g. AWS D1.1 Annex B uses its own H1 / H2 / H3 hydrogen levels (on the order of ≤ 5, ≤ 10, ≤ 16 ml/100 g respectively). Entering the nomogram without this conversion is a classic calculation error.
Redrying and storage: Redry low-hydrogen covered electrodes at the temperature the manufacturer specifies, then keep them in a portable electrode holding oven (quiver, typically 120–150 °C). On work under AWS D1.1, store electrodes in an oven held at a minimum of 120 °C [250 °F] once the hermetic packaging is opened or the electrodes leave the oven. A lower quiver temperature is a code violation. Consume electrodes within the atmospheric exposure time limit allowed by the code and the manufacturer.
Joint cleanliness: Rust, oil, paint, moisture, and condensation will void even the best electrode management.

3. Cold (hydrogen) cracking: when four conditions meet

Cold cracking, also known as delayed cracking or hydrogen-induced cracking, appears when these conditions come together:

Susceptible microstructure — HAZ or weld metal containing high-hardness martensite,
Diffusible hydrogen — hydrogen dissolved in the pool and later trapped,
Tensile stress — residual stresses from shrinkage and restraint,
Low temperature window — the structure dropping below roughly 150–200 °C; hydrogen cracking does not occur at high temperature.

Reduce any one of these enough and the cracking risk falls to an acceptable level. In practice you do not zero the risk out; you manage it. Preheat hits three fronts at once: it lowers the cooling rate and thus the hardness, it gives hydrogen time to escape, and it delays the descent into the critical temperature window.

The crack does not always sit in the HAZ. In modern high-strength (≥ 690 MPa yield) and low-carbon TMCP steels, the dominant cracking location is the weld metal. As base metal carbon drops, HAZ hardenability drops with it. The weld metal, on the other hand, completes its transformation later and therefore traps hydrogen inside itself. In that case the crack typically shows up as transverse or chevron morphology, directly in the weld metal. Selecting a filler metal stronger than the joint needs (overmatching) raises the risk markedly, which makes filler metal selection a cracking-risk decision, not just a strength decision.

The "delayed" nature of the crack is its most treacherous feature. The weld can look sound at the time of testing, while hydrogen keeps accumulating by diffusion at critical points over anything from a few hours to 48 hours, sometimes longer. That is why the waiting period comes from the code. AWS D1.1 allows nondestructive testing on ASTM A514, A517, and A709 Grade 100 / 100W steels to start once the weld has cooled to ambient temperature, but it requires the acceptance criteria to rest on testing performed no earlier than 48 hours after welding is complete. For other steels, cooling to ambient temperature may be considered sufficient, while many pressure equipment specifications call for a 24–48 hour wait on high-strength steels. There is no "general 48 hour rule". Check the project specification and the code.

The choice of testing method also becomes critical here. A significant share of delayed cracks in the HAZ and in the weld metal are underbead cracks that never break the surface, and MT cannot see them. Delayed cracks that do not reach the surface call for volumetric testing (UT, and where suitable PAUT or TOFD). MT alone is not adequate assurance.

4. Carbon equivalent: mapping the hardening tendency

How much preheat a steel needs depends largely on its chemical composition. Carbon does not decide it alone: manganese, chromium, molybdenum, vanadium, nickel, and copper all raise hardenability too. Carbon equivalent collects those effects into a single number — but there is no single carbon equivalent formula, and the formulas are not interchangeable.

IIW / CEV (the classic relation): CEV = C + Mn/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

The higher the CEV, the stronger the tendency to form martensite. As a rough practical threshold, CEV above roughly 0.40 counts as "watch the preheat" territory, and that is never an absolute limit. The threshold does not stand apart from thickness either: a thin section may need no preheat even at CEV 0.45, while a 50 mm combined thickness may need it at CEV 0.38. CEV alone never decides preheat. Always enter the nomogram with combined thickness, heat input, and HD level alongside it. The IIW relation gives meaningful results on steels with carbon content above roughly 0.18%.

AWS D1.1 relation (Annex B): CE = C + (Mn+Si)/6 + (Cr+Mo+V)/5 + (Ni+Cu)/15

Note the added silicon term. One very common conceptual mix-up needs clearing up here: in AWS D1.1 Annex B, CE is the input to the HAZ hardness control method only. The input to the hydrogen control method is not CE. It is the susceptibility index derived from Pcm and the hydrogen level H (Susceptibility Index Grouping, A–G). Entering the D1.1 HAZ hardness control tables with a value calculated from the IIW formula gives a wrong answer, and treating CE as the input to the susceptibility index mixes the two methods together. Whichever code and method you work to, use that method's own formula.

Method selection carries one more boundary: the HAZ hardness control method applies to fillet welds only; you cannot use it on groove (butt) welds. For groove welds, go to the hydrogen control method.

Pcm (Ito–Bessyo cracking parameter): Pcm = C + Si/30 + (Mn+Cu+Cr)/20 + Ni/60 + Mo/15 + V/10 + 5B

Pcm is not an AWS formula. Ito and Bessyo (Nippon Steel, 1968) developed it, and it was later standardized within JIS / WES 3001. AWS D1.1 uses it in the Annex B hydrogen control method. On low-carbon steels (roughly C ≤ 0.18%, including modern TMCP products) it discriminates more sensitively than the IIW relation.

CET (EN 1011-2, Method A): CET = C + (Mn+Mo)/10 + (Cr+Cu)/20 + Ni/40

EN 1011-2 Annex C gives two separate methods for the preheat calculation:

Method A — CET based. Developed mainly for low-alloy high-strength steels. Inputs: CET, plate thickness d, HD (directly in ml/100 g), and heat input.
Method B — CEV based. A nomogram approach with roots in BS 5135, aimed mainly at C-Mn steels. Inputs: CEV, combined thickness, the A–E hydrogen scale, and heat input.

The method-to-letter mapping is fixed in the standard; it does not shift from one edition to the next. One real risk does deserve attention, though: entering the right nomogram with the wrong carbon equivalent is a classic field error.

The CET method is a closed empirical relation, and the result means nothing outside its validity range. EN 1011-2:2001 Annex C sets these limits:

CET = 0.20–0.50%
Thickness d = 10–90 mm
HD = 1–20 ml/100 g
Heat input Q = 0.5–4.0 kJ/mm
Yield strength ≤ 1,000 N/mm²

So applying this formula on an 8 mm plate or to a 6 kJ/mm SAW pass is invalid. In those cases, go to Method B or qualify by test.

The target of preheat is not a temperature; it is a hardness and microstructure result. Temperature is only the tool, and the measurable output is hardness. The HAZ hardness control method in AWS D1.1 Annex B picks the critical cooling rate straight from a target HAZ hardness: 350 HV in the general case, and 400 HV where low-hydrogen filler metal is used. On sour service (H₂S bearing) lines, the NACE MR0175 / ISO 15156 limit is far stricter: ≤ 250 HV10 (≈ 22 HRC). Practical summary: typical target on structural work ≤ 350 HV10, on sour service lines ≤ 250 HV10. Qualification is what finally confirms that the preheat was chosen correctly — backed up, where required, by field replication or portable hardness measurement.

5. Thickness, restraint, and combined thickness

The same steel may weld without preheat at 8 mm and demand serious preheat at 40 mm. The reason is that a thick section behaves like a heat sink: heat spreads fast in three dimensions, the weld cools much faster, and t8/5 shortens. That is why the AWS D1.1 prequalified minimum preheat and interpass temperature table (Table 5.8) grades its values not only by material group but also by the thickness of the thickest part being joined.

It is worth stressing that this table is not mandatory: Table 5.8 binds only when you take the prequalification route (Clause 5, Part F). If the WPS is qualified by test (Clause 6), the preheat value rests on the PQR. If you use the Annex B alternative method, you may be able to justify a lower preheat than the table shows — and equally, under high restraint or a high hydrogen level, the method may produce a value higher than the table.

Thickness also raises restraint. A thick, rigid assembly resists shrinkage, and that means higher residual tensile stress in the HAZ. Calculations account for the thicknesses of all parts meeting at the joint as the combined thickness. In EN 1011-2, combined thickness is the sum of the thicknesses of the parts entering the joint, measured 75 mm from the weld line. How many parts you sum depends on the joint type: two parts in a butt joint, three in a T-joint (web plus the flange section on either side), and the relevant sections again in a lap joint. In short, the preheat decision is a joint function of three variables: chemistry (CEV/CET/Pcm) + thickness and restraint + hydrogen level (process and consumable).

Add ambient conditions on top of that. AWS D1.1 carries two separate rules, and they must not be confused:

If the base metal is below 0 °C, remove condensation and ice, heat the base metal to at least 20 °C, and maintain that temperature throughout welding.
Do not weld while the ambient temperature is below −18 °C. This condition applies to the surroundings, not to the base metal, and you may take the reading inside a heated shelter that encloses both the welder and the work.

6. The interpass upper limit: capping matters as much as heating

People usually think of preheat and interpass temperature as a "lower limit", yet interpass temperature also has an upper limit. In multi-pass welds every new pass adds heat to the passes below it. Exceed the upper limit without control and you get this:

The cooling rate slows excessively and t8/5 stretches too far. The main drivers of grain coarsening are the peak temperature and the time spent above roughly 1,100 °C; high heat input and rising interpass temperature together increase grain coarsening in the HAZ.
Toughness drops in the weld metal and the HAZ, and low-temperature Charpy impact performance suffers in particular.
In quenched and tempered (Q&T) and TMCP steels, approaching the tempering temperature causes strength loss from overtempering.
In duplex and super duplex stainless steels, the real reason is phase balance. Repeated thermal cycling upsets the ferrite/austenite balance and leads to secondary austenite formation, which lowers both toughness and corrosion resistance. On top of that, sigma (σ) and chi (χ) intermetallic phases precipitate in roughly the 600–1,000 °C range, depending on the time spent there. Lower down, 475 °C embrittlement (alpha-prime precipitation, roughly 300–525 °C) takes over, and that is the second, often overlooked reason for keeping the interpass ceiling low. In austenitic stainless steels the main risks are instead hot cracking and chromium carbide precipitation (sensitization).

Work through the ceiling family by family. The values below appear frequently in project specifications (they are common WPS practice, not a binding code clause): on C-Mn steels the ceiling most often sits at 250 °C; on toughness-critical work and on Q&T and TMCP steels, usually ≤ 200 °C; on standard/lean duplex stainless steels (such as 2205), typically around 150 °C; on super duplex (25Cr) stainless steels, most specifications cap it at ≤ 100 °C, along with a heat input usually limited to ≤ 1.5 kJ/mm. The binding value is always the WPS and the project specification. So the inspector asks two questions together: "is it hot enough?" and "is it too hot?".

7. Measurement: where, when, how

The right measurement matters as much as the right value. EN ISO 13916 defines the measurement of preheat temperature, interpass temperature, and preheat maintenance temperature. Watch that third term: it is the temperature you hold the joint at after welding stops, not a reduced preheat.

Measurement location (EN ISO 13916): Measure the temperature on the surface the welder faces, at a distance A from the edge of the weld preparation:

For thickness t ≤ 50 mm, A = 4 × t, up to a maximum of 50 mm.
For thickness t > 50 mm, achieve the required temperature within at least 75 mm of the preparation (or a distance agreed between the parties).

The 50 mm here is an upper limit, not a lower one. For example, on 22 mm pipe wall thickness you calculate 4 × 22 = 88 mm, apply the cap, and the measurement point lands at 50 mm.

Do not confuse this rule with the heated-zone rule in AWS D1.1: under D1.1, maintain preheat and minimum interpass temperature in all directions from the point of welding for a distance equal to the thickness of the thickest part, but not less than 75 mm. That defines the area the heating must cover, not a measurement point.

Timing: Take the measurement after the heat has equalized through the section. Read from the face opposite the heated side where possible. If you must read from the heated face, withdraw the heat source first and allow an equalization time of roughly 2 minutes per 25 mm of base metal thickness as the standard's guidance value (about 2 minutes for 22 mm). That value is informative guidance in EN ISO 13916; the binding time comes from the WPS or the project specification.

Interpass: Measure immediately before starting the next pass, from the same measurement location (distance A) used for preheat. Note that the two codes ask different questions here: EN ISO 13916 ties the lower-limit measurement to distance A, while on the ASME Section IX side the maximum interpass temperature recorded governs the upper-limit check (QW-406.3), so in practice the hottest region decides. Unless the WPS says otherwise, base the lower-limit check on distance A. A random reading off the edge of the weld misleads you in either case.

Tools:

Thermocouple: The most reliable method, and it can be recorded. On critical work this is the reference.
Temperature-indicating crayon (e.g. Tempilstik): Shows only whether one threshold has been crossed. It is not sufficient on its own for maximum interpass control (the upper limit needs a separate crayon or a thermocouple), and crayon residue smeared on the weld surface can create contamination. On stainless, duplex, and nickel alloys, many specifications ban crayons containing chloride, sulfur, or low-melting-point metals outright (risk of stress corrosion cracking and low-melting-point metal embrittlement). On those materials use only certified low-halogen/low-sulfur types, and even then only at distance A and off the weld.
Non-contact pyrometer / thermal camera: Practical, but sensitive to surface emissivity; oxide, paint, and surface brightness shift the reading. The second error source is the distance-to-spot ratio (D:S) — hold the instrument far away and the measured area grows, averaging in the cold surface around the target. The third is failing to correlate the instrument against a thermocouple. Practical rule: correlate non-contact instruments against a thermocouple at the start of every shift and keep the calibration record.

Applying the heat: Use flame, electrical resistance blankets, or induction. Make the heating band wide enough on both sides of the weld preparation and, where possible, uniform around the circumference. In local flame heating, the biggest error source is heating the surface while the section stays cold.

8. Control and records: where the procedure meets the field

Preheat and interpass values are not arbitrary. They rest on a procedure prepared to EN ISO 15609-1 (WPS for arc welding) or ASME BPVC Section IX, and qualified under EN ISO 15614-1 or Section IX.

The variable classification matters. In ASME BPVC Section IX:

A decrease of more than 55 °C (100 °F) in the qualified preheat temperature is an essential variable (QW-406.1).
An increase of more than 55 °C in the maximum interpass temperature is a supplementary essential variable (QW-406.3), and it applies only where notch toughness requirements are specified.
QW-406.2 is not about this: that paragraph covers the change in maintaining (or decreasing) preheat at the completion of welding and prior to PWHT, and for most processes it is a nonessential variable. Mixing the two paragraphs is a common mistake.

On the EN ISO 15614-1 side, you may not go below the qualified minimum preheat, and you may not exceed the qualified maximum interpass temperature.

The inspector's practical checklist:

Is the WPS minimum preheat achieved before the first pass starts?
Is the measurement taken from the correct location (4t, up to 50 mm) and with the correct tool?
Is the equalization time allowed (roughly 2 minutes per 25 mm as the standard's guidance value)?
Does everyone know that the minimum preheat is also the minimum interpass temperature?
Are tack welds, arc strikes, temporary attachments, lug welds, and thermal gouging carried out at the same minimum preheat? Tack welds, with their low heat input and high cooling rate, are the most classic starting point for hydrogen cracking.
On multi-pass welds, is the interpass temperature checked before every pass, and is the upper limit respected?
When welding is interrupted and the part cools, is preheat reapplied?
Is preheat maintained throughout welding, and is the heating band wide enough?
If post-heating (DHT) is required, does it start before the part drops below the preheat temperature?
Does the consumable HD class (H5/H10/H15 in EN, H4/H8/H16 in AWS) match the WPS, and are redrying, quiver temperature (≥ 120 °C), and exposure time recorded?
Is a hardness target defined (≤ 350 HV10 on structural work, ≤ 250 HV10 on sour service lines) and verified?
Are the values entered on the weld travel sheet / thermal record? (Traceability matters for EN ISO 3834-2 quality requirements.)

Correct heating carried out without records counts as "not done" in an audit. Documentation is as much part of the job as measurement.

From the field

On a process plant project, a pressure piping spool under ASME B31.3 had a heavy-wall joint — 22 mm pipe wall thickness — and the WPS called for 100 °C preheat. (The project specification had adopted EN ISO 13916 as the contractual measurement method. On pipeline girth welds the governing code is usually API 1104 or ASME B31.4/B31.8, and the measurement provisions differ, so settle which code governs on day one.) The crew was applying preheat and checking it with a temperature-indicating crayon, but they took the reading straight off the weld preparation, on the spot the flame had just passed. EN ISO 13916 puts the measurement point at 50 mm from the edge of the preparation here: 4 × 22 = 88 mm, so the 50 mm cap applies. A thermocouple check at that point read only 60–65 °C. The surface looked "hot" while the section had never warmed through.

Forty-eight hours after the root pass, the MT + UT re-test required by the project specification found fine, longitudinal delayed cracks in the HAZ. Some of the indications never broke the surface, and only UT picked them up — the classic hydrogen cracking signature. The fix took four steps. First, move the measurement point to comply with the 4t / 50 mm maximum rule. Second, allow the roughly 2 minutes per 25 mm equalization time, since the reading came from the heated face. Third, switch to a low-hydrogen (H5) electrode for every pass including the root, carry it in a portable electrode holding oven (quiver, ≥ 120 °C) after redrying, and consume it inside the manufacturer's atmospheric exposure time limit. Fourth, prove the fix worked with portable hardness measurement — HAZ hardness came in below the 350 HV10 target on the test coupon and on the production weld alike. After those corrections, the same procedure produced zero cracks. The lesson: a correct number measured in the wrong place is a wrong number, and the proof of the fix is hardness, not temperature.

Related standards

EN ISO 13916:2017 (new edition: 2025) — Measurement of preheat, interpass, and preheat maintenance temperatures; measurement location (A = 4t, up to 50 mm, for t ≤ 50 mm) and equalization time guidance values.
AWS D1.1/D1.1M:2020 and :2025 — Structural welding code for steel; Table 5.8 gives minimum preheat/interpass values for prequalified WPSs, and Annex B gives the alternative methods: (a) the HAZ hardness control method — fillet welds only, input CE; (b) the hydrogen control method — input Pcm plus the susceptibility index derived from the hydrogen level, and the restraint level. Note: in editions before 2020 this annex was numbered Annex XI (and Annex H in older editions); do not cite the old annex letters, and confirm clause and table numbers against the edition in your hands.
EN ISO 15609-1:2019 — Welding procedure specification (WPS) for arc welding; documenting preheat and interpass values.
EN ISO 15614-1:2017+A1:2019 / ASME BPVC Section IX (current edition) — Procedure qualification. In Section IX, a decrease of more than 55 °C in preheat is an essential variable (QW-406.1); an increase of more than 55 °C in the maximum interpass temperature is a supplementary essential variable (QW-406.3); QW-406.2 is a separate variable covering the maintenance/decrease of preheat.
EN 1011-2:2001+A1:2003 — Recommendations for welding ferritic steels; Annex C gives Method A (CET based) and Method B (CEV based) preheat calculation methods, t8/5, combined thickness, and the hydrogen scale. EN 1011-1 gives the definition of heat input and the k efficiency factor (together with ISO/TR 18491).
EN ISO 3690:2018 — Determination of diffusible hydrogen in weld metal; the measurement basis for the H5/H10/H15 classification.
EN ISO 3834-2:2021 — Comprehensive quality requirements for welding; the framework for controlling and recording thermal parameters.
NACE MR0175 / ISO 15156 — Materials requirements for sour (H₂S) service; the ≤ 250 HV10 (≈ 22 HRC) hardness limit for the weld and the HAZ.

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