What does NACE MR0175 require for machined parts in sour service, and how do you verify hardness compliance?

When does NACE MR0175 apply to a machined part, and who decides?

It applies when the equipment user or the purchaser specifies it for a part that will be wetted by an H2S-containing production fluid. The machine shop does not decide that a part is in sour service; the drawing, purchase order or project specification has to say so.

The standard is published in three parts. ISO 15156-1 sets the general principles and places the decision on whether the service is sour, and how severe it is, with the equipment user. ISO 15156-2 covers carbon and low-alloy steels and cast irons, and ISO 15156-3 covers corrosion-resistant alloys such as stainless steels and nickel alloys. Per ISO 15156-2, carbon and low-alloy steels normally need no special sulfide stress cracking precautions when the H2S partial pressure in the gas is below 0.3 kPa (0.05 psi); at or above that, SSC-resistant steels are required. That threshold is for the equipment user to apply to the process conditions, not something a supplier can check from a drawing.

Typical machined parts that carry the requirement are valve stems, bonnets, seats and plugs, instrument fittings and manifolds, pump shafts, wellhead components, bolting, and the housings and adapters around them. The standard addresses cracking only. It does not cover general corrosion or pitting, so a part can comply with NACE MR0175 and still corrode in service if the alloy is wrong for the chloride or CO2 content of the fluid.

The practical consequence for a buyer is simple: if the requirement is not on the purchase order, the supplier has no reason to buy controlled bar or test hardness, and an otherwise good part will be rejected the moment someone downstream asks for proof.

Why do machined parts get rejected for sour-service hardness or material?

The common causes, roughly in order of how often they show up, are bar bought in a hardened or cold-finished condition, a free-machining or high-nickel grade substituted, the wrong heat-treated condition on a stainless or 17-4 PH part, and hardness evidence the customer cannot accept. Welds and local cold work are the next most common trap.

1. Pre-hardened or cold-finished bar. Pre-hardened 4140 bar is popular in machine shops because it needs no heat treatment after machining, but it is typically supplied well above the 22 HRC limit set per ISO 15156-2. Cold-drawn carbon bar can also run harder than the hot-rolled grade it replaces. Same alloy, wrong condition, rejected part.
2. A free-machining substitute. 12L14, 1215, 303 and 416 are chosen for machinability and are excluded from sour service. A supplier who swaps 303 for 304 to save cycle time has produced scrap, even if every dimension is perfect.
3. Nickel content. Low-alloy steels such as 4340 carry nickel typically well above the 1 % ceiling set per ISO 15156-2 for carbon and low-alloy steels, so they fail on chemistry before hardness is even tested.
4. Wrong heat-treated condition. A 17-4 PH part aged to H900 or H1025 for strength cannot meet the sour-service condition, and a 316 part made from strain-hardened bar is not in the solution-annealed condition the standard expects.
5. Welds, repairs and local cold work. A weld repair, a welded-on boss or hard-facing adds a heat-affected zone that needs its own procedure and hardness survey. Cut threads, rolled threads and light burnishing are generally acceptable on material that otherwise complies, but per ISO 15156-2 cold straightening or forming that leaves more than 5 % permanent outer-fiber strain on carbon or low-alloy steel calls for a thermal stress relief, and cold work done to raise strength takes austenitic stainless out of its acceptable condition.
6. Evidence that does not prove anything. A mill certificate with no heat-treated condition, a hardness value in a scale the customer did not ask for, a test taken on the bar rather than the part, or a heat number that cannot be traced to the lot.

How do you fix a lot that failed sour-service hardness, and how do you prevent it?

First establish whether the failure is chemistry, condition or evidence, because only the last two can be fixed on existing parts. A wrong grade is scrap; a wrong condition can sometimes be corrected by an approved heat treatment; missing evidence can be produced by testing.

Any rework heat treatment is a new process step. It changes the dimensions and surface condition of a finished part, it needs its own record, and it should be approved in writing by the customer, who may also want a fresh hardness survey and a nonconformance report. Prevention is cheaper: the decisions that avoid most rejections are made when the bar is ordered, not when the part is inspected.

How is sour-service material control handled on a Saola order?

CNC machining runs at our lead CNC plant. Heat treatment and any finishing go to managed partner plants, and each quote names the production site and the country of origin.

We work to the material, hardness and evidence requirements on your purchase order; whether a part is fit for your sour environment stays the equipment user’s decision.

Turned and milled parts are made on the CNC turning and CNC milling equipment at T.K Viet Nam Precision Company Limited, our lead CNC plant since 2017, which has a hardness tester for incoming bar and finished-part checks. When the RFQ names NACE MR0175/ISO 15156, the quote states which grade and condition will be bought, what evidence comes with the parts, and whether an outside laboratory is needed for any test you require. Heat treatment, aging and any coating are separate steps at partner plants, so they are listed on the quote with their records. ISO 9001 certification is in progress, not held. If the bar you need is not routinely stocked, we say so on the quote rather than offering a substitute grade.

Send the drawing, the sour-service note and your evidence requirements through the quote form.