H1150D: The Only NACE-Compliant 17-4PH Condition for Sour Service

Q: Is H1150D the same as H1150M?

No. H1150D is two identical 621 C (1150 F) aging cycles, the double-age used for sour service. H1150M uses a high first stage near 760 C (1400 F) followed by a 621 C stage, and is aimed at maximum toughness in large rotating forgings such as turbine impellers. For H2S wellhead work, specify H1150D.

Q: What yield strength does H1150D still deliver?

Around 725 MPa (105 ksi) minimum yield, which is exactly why it aligns with the API 6A 105K material class. You trade the very high strength of H900 for the toughness and hydrogen resistance that H2S service requires.

Q: Does H1150D work below -46 C?

API 6A Class L qualifies -46 C. Below that, for example -60 C arctic wellheads, impact behavior must be individually qualified; VAR-refined H1150D or an H1150M variant is usually the better route. Supplemental Charpy testing at the project temperature should be run during first-article qualification.

Quick answer

H1150D is the only 17-4PH heat-treatment condition qualified for H₂S sour service under NACE MR0175 / ISO 15156. It applies two identical aging cycles at 621°C (1150°F), which over-ages the martensite and keeps hardness at or below 33 HRC — the threshold above which sulfide stress cracking (SSC) becomes a risk. Harder conditions (H900–H1025) exceed 33 HRC, and single-aged conditions (H1075–H1150) are not the treatment the standard qualifies, even when they test under 33 HRC.

Key takeaways

NACE MR0175 / ISO 15156-3 caps 17-4PH (UNS S17400) at 33 HRC and qualifies it only in the double-aged condition.
H1150D = two 621°C aging cycles (API 6A Method A); it typically lands at ~24–28 HRC with a safe margin below the limit.
The second age completes over-aging, tempers the martensite, and forms reverted austenite that traps hydrogen — the real reason it resists SSC.
Hardness alone isn't enough: sour-service forgings also need ESR or VAR melting to remove inclusions that initiate cracks.
Compliance is proven with per-piece hardness, Charpy impact (API 6A Class L/P), 100% NDE, and an EN 10204 3.1/3.2 certificate.

When a 17-4PH forging fails in a sour well, it rarely fails because the steel was weak. It fails because the steel was too strong. Sulfide stress cracking punishes hardness, and the single specification decision that separates a compliant wellhead forging from a liability is the aging condition. For H₂S service, that decision has exactly one right answer: H1150D — the double-aged condition.

This article explains why — not the marketing version, the metallurgical one: what the second aging cycle changes inside the martensite, why a hardness number alone doesn't make a forging "sour-ready," and how a supplier proves the condition on paper. Everything below applies to 17-4PH (UNS S17400 / AISI 630) forgings supplied for oil & gas wellhead, valve, and subsea equipment.

01 The failure mode

What H₂S actually does to a hardened forging

"Sour service" means the produced fluid contains hydrogen sulfide above a threshold partial pressure. The corrosion reaction at the steel surface liberates atomic hydrogen, and H₂S is a potent "poison" that stops those hydrogen atoms from recombining into harmless H₂ gas. Instead, they diffuse into the metal.

Inside a hard, highly stressed martensitic structure, that dissolved hydrogen collects at grain boundaries, inclusions, and the tips of tiny flaws. Under sustained tensile load it drops the stress needed to crack the material — often far below the yield strength. The result is sulfide stress cracking (SSC): sudden, brittle, and with almost no warning. It is a specific form of hydrogen-assisted cracking, and its single strongest driver is hardness.

The core relationship

Susceptibility to SSC rises steeply with hardness. Above roughly 33 HRC, martensitic PH stainless steels can no longer be relied upon to resist cracking in H₂S. Below it, with the right microstructure, they can. That threshold is not a Jiangsu Liangyi rule — it's written into the international standard.

02 The governing standard

The one number that governs everything: 33 HRC

NACE MR0175 / ISO 15156-3 is the standard that qualifies metals for H₂S service. For 17-4PH (S17400) it does two things at once, and both matter:

It caps hardness at 33 HRC maximum — verified, not assumed.
It qualifies the alloy in one specified heat-treatment condition — the double-aged condition — rather than "any condition that happens to test under 33 HRC."

That second point is the one procurement teams miss most often. Meeting the hardness number is necessary, but on its own it is not sufficient. Two forgings can read the same HRC and behave completely differently under hydrogen depending on the path their microstructure took. Double aging is the qualified path.

Naming note

"H1150D," "Double H1150," and "H1150 × 2" all describe the same treatment: two full aging cycles at 1150°F (621°C). In API 6A this is Method A double-aging. It is distinct from H1150M, which uses a high first stage (~760°C) followed by a 621°C stage.

03 Condition-by-condition

Where every 17-4PH aging condition lands

Order the standard conditions by hardness and the argument becomes visual. Three lower conditions blow straight through the 33 HRC ceiling. Three more slip under it but were never the qualified sour-service treatment. Only one satisfies both requirements.

H900482°C

✕ ~40 HRC · over limit

H925496°C

✕ ~38 HRC · over limit

H1025552°C

✕ ~35 HRC · over limit

H1075579°C

△ ~32 HRC · single-aged only

H1100593°C

△ ~31 HRC · not the spec'd condition

H1150621°C ×1

△ ~28 HRC · single age ≠ qualified

H1150D621°C ×2

✓ ≤33 HRC · double-aged · compliant

33 HRC — the NACE MR0175 / ISO 15156 ceiling for 17-4PH

Exceeds the hardness limit Under limit, but not the qualified treatment Double-aged & compliant

04 The metallurgy

Why the second aging cycle is the whole point

17-4PH gets its strength from copper-rich precipitates that form in the martensite during aging. Age low and short (H900, 482°C) and those precipitates are fine and dense — peak hardness, peak strength, and peak SSC risk. Age hotter (621°C) and the picture changes deliberately in three ways:

The copper precipitates coarsen and over-age. They grow and space out, which drops hardness and strength on purpose — the trade the standard wants.
The martensite tempers. Carbon redistributes and internal residual stresses relax, removing the brittle, highly-strained matrix that hydrogen exploits.
Reverted austenite forms. Fine, stable austenite appears at 621°C. It is tougher, and — importantly for sour service — it acts as a benign trap for hydrogen, keeping it away from the crack-prone boundaries.

A single 621°C age (H1150) already moves in the right direction. So why does the standard insist on a second identical cycle? Because one pass leaves the job half-finished and, worse, inconsistent:

Uniformity. The second cycle drives coarsening and reverted-austenite formation to completion through the whole section — surface and core alike. In a heavy forging, a single age can leave localized zones still above 33 HRC. Double aging pulls the whole part comfortably under the ceiling.
Stability. The repeat cycle stabilizes the reverted austenite and further relaxes residual stress, so properties don't drift and the toughness gain is real, not marginal.
Margin. H1150D typically lands around 24–28 HRC — well below 33, not flirting with it. That margin is what survives real-world scatter across a production lot.

Net effect of the second age

Lower, more uniform hardness · tempered and de-stressed martensite · stable reverted austenite that traps hydrogen. That combination — not the hardness number by itself — is what resists sulfide stress cracking.

05 The recipe

H1150D, step by step (API 6A Method A)

The sequence below is a real, ordered process — each step depends on the one before it. This is the double-aging protocol we run on our sour-service 17-4PH forgings.

Solution anneal — 1026–1050°C (1875–1925°F)

Hold at least one hour, then quench in air or nitrogen to ≤32°C (90°F). This dissolves the copper into solution and resets the microstructure to a clean starting point.

First age — 621°C ±14°C (1150°F ±25°F)

Hold ≥4 hours, then quench to ≤32°C. Over-ages the copper precipitates and begins tempering the martensite and forming reverted austenite.

Second age — 621°C ±14°C, repeat

Repeat the identical 1150°F soak for ≥4 hours, quench to ≤32°C. This is the cycle that makes it "H1150D" — completing coarsening, stabilizing austenite, and locking hardness below 33 HRC through the full section.

Large-section practice

On forgings over ~200 mm section, we add a slow furnace cool (≤56°C/h from 204°C to ambient). It isn't required by ASTM A564, but it eliminates the thermal-stress gradients that cause hardness and toughness to vary between surface and core.

06 Beyond heat treatment

The right condition isn't enough on its own

Double aging fixes hardness and microstructure. It does nothing about the flaws hydrogen loves most: non-metallic inclusions. Sulfide and oxide inclusions are ready-made initiation sites for SSC, and no aging cycle removes them. That's why melt route belongs in the same conversation as heat treatment.

ESR (Electroslag Remelting) refines the ingot through a molten slag layer, stripping out most non-metallic inclusions. It's the right baseline for the large majority of API 6A 105K sour-service work.
VAR (Vacuum Arc Remelting) goes further, removing dissolved gases under vacuum for the cleanest, most homogeneous structure — specified for the most critical or highest-H₂S applications.

Air-melted 17-4PH, even correctly double-aged, is a non-sour material. For H₂S service, specify ESR as a minimum and reserve VAR for the severe cases. Getting H1150D right on dirty stock is a false economy.

"Three previous suppliers had failed our QC on Charpy or hardness before the parts even shipped. The certificate said H1150D — the material didn't behave like it." — A recurring pattern in sour-service RFQs

07 Proof, not promises

How compliance is actually verified

"H1150D" on a purchase order means nothing without the tests that back it. A sour-service forging should arrive with proof of three things, each on paper:

Table 1 — API 6A 105K Charpy V-notch impact requirements (ASTM A370)
Temperature class	Test temp.	Min. average	Min. individual
Class L (arctic)	−46°C (−50°F)	27 J	18 J
Class P (standard)	−29°C (−20°F)	41 J	27 J

Hardness on every piece. Not batch sampling — each forging read and recorded ≤33 HRC after double aging.
Charpy impact per API 6A class. Well-run H1150D routinely delivers 45–52 J at −29°C, comfortably clear of the 41 J Class P average.
Full NDE. 100% ultrasonic testing (ASTM A388) plus magnetic particle examination (ASTM A275) to catch the sub-surface flaws that seed cracking.

All of it is captured on an EN 10204 3.1 certificate — or 3.2 when a third party (e.g. Bureau Veritas, SGS, DNV, TÜV or Lloyd's Register) witnesses the tests — with a NACE MR0175 / ISO 15156 compliance statement and per-heat traceability. If a supplier can't produce those hardness records piece by piece, the "H1150D" claim is unverified.

08 Prohibited vs. qualified

The short version, in one table

Table 2 — 17-4PH conditions for H₂S sour service
Condition	Typical HRC	Aging	Sour service
H900	~40	single, 482°C	Prohibited — too hard
H925	~38	single, 496°C	Prohibited — too hard
H1025	~35	single, 552°C	Prohibited — too hard
H1075	~32	single, 579°C	Not qualified — single-aged
H1100	~31	single, 593°C	Not qualified — single-aged
H1150	~28	single, 621°C	Not qualified — needs double age
H1150D	≤33 (≈24–28)	double, 621°C ×2	✓ Compliant — the only one

09 Engineering FAQ

Questions procurement teams ask us

H1075 tests under 33 HRC — why can't we use it for sour service?

Because NACE MR0175 / ISO 15156 qualifies 17-4PH in the specified double-aged condition, not "any condition that reads under 33 HRC." A single age can leave localized hard zones and a less stable microstructure, and it lacks field-service history under the standard. Hardness is a pass/fail gate, not the qualification itself.

Is H1150D the same as H1150M?

No. H1150D is two identical 621°C (1150°F) aging cycles — the double-age used for sour service. H1150M uses a high first stage near 760°C (1400°F) followed by a 621°C stage, and is aimed at maximum toughness in large rotating forgings such as turbine impellers. For H₂S wellhead work, specify H1150D.

Can double aging alone make air-melted 17-4PH sour-ready?

No. Aging controls hardness and microstructure but not inclusion cleanliness. Sulfide and oxide inclusions initiate cracking regardless of condition. Specify ESR (minimum) or VAR remelting for sour-service forgings; air-melted material is for non-sour use.

What yield strength does H1150D still deliver?

Around ≥725 MPa (105 ksi) minimum yield — which is exactly why it aligns with the API 6A "105K" material class. You trade the very high strength of H900 for the toughness and hydrogen resistance that H₂S service requires.

Does H1150D work below −46°C?

API 6A Class L qualifies −46°C. Below that (e.g. −60°C arctic wellheads), impact behavior must be individually qualified; VAR-refined H1150D or an H1150M variant is usually the better route. We run supplemental Charpy at your project temperature during first-article qualification.

About the author

Jiangsu Liangyi Co., Limited is an ISO 9001-certified manufacturer of 17-4PH (UNS S17400 / AISI 630) forgings, sour-service forged bars produced to API 6A 105K requirements, and seamless rolled rings, with 20+ years of production for oil & gas, power generation, marine and valve customers in 50+ countries. This brief reflects our standard H1150D practice and delivery documentation.

Reference standards: NACE MR0175 / ISO 15156-3 · API Specification 6A (105K) · ASTM A564 / A705 · ASTM A370 · ASTM A388 · ASTM A275 · EN 10204. Values are typical for guidance; qualify against your project specification.

Need 17-4PH forgings that are genuinely H1150D-qualified?

Sour-service bars, rings and custom forgings produced to API 6A 105K requirements — double-aged, ≤33 HRC verified on every piece, ESR/VAR refined, with NACE MR0175 statements on EN 10204 3.1/3.2 certificates.

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