Related Specifications
Surface Treatments
Certifications
- ISO 9001 - 2015 Certified
- PED 2014/68/EC
- API 6A ; API 20E; API 20F
- NACE MR0175/ISO 15156-2
- NORSOK M-650
- DFAR
- MERKBLATT AD 2000 W2/W7/W10
B8M Chloride SCC Resistance: When 316 Bolting Cracks and What to Specify Instead
ASTM A193 Grade B8M is the 316 stainless variant of the A193 family (UNS S31600, Cr 16-18%, Ni 10-14%, Mo 2.00-3.00% per ASTM A193/A193M Table 1). It is the bolting most commonly specified for chloride-bearing flange service, yet it is also the bolting most commonly recovered from chloride-induced cracking failures. The molybdenum addition raises pitting resistance over plain B8 but it does not move B8M into the class of alloys that are immune to chloride stress corrosion cracking. This page sets out the service window where B8M behaves, the threshold where it does not, and the upgrade ladder operators should specify when the duty is harsher than B8M can carry.
Need a chloride-service bolting recommendation for a specific flange, valve, or vessel? Email TorqBolt QA-QC with the temperature, chloride concentration, and bolt size and we will return a grade-and-class proposal within one working day.
What chloride SCC actually is in a bolted joint
Chloride stress corrosion cracking (Cl-SCC) is a transgranular, branched-crack failure mechanism that needs three ingredients on the bolt surface at the same time: a tensile stress, an aqueous chloride film, and a temperature high enough to mobilise the chloride ions against the passive oxide layer. Pull any one of those three legs away and SCC stops. In a flange joint the tensile stress is always present because the bolt is preloaded against the gasket. The chloride film is present whenever the joint sees insulation sweat, salt-laden marine air, produced water, or process leakage. So the controllable lever in service is temperature, and that is the lever the chloride SCC threshold tables are built around.
In austenitic stainless steels the classical industry threshold cited in ASM Handbook Volume 13 and in countless plant failure reports is in the region of 60 °C bulk metal temperature combined with around 100 ppm dissolved chloride. Below that combined window Type 316 (and therefore B8M) is generally serviceable. Above it the risk rises sharply, and under insulation, where the actual surface temperature can run 20 to 30 °C above the process temperature on a cycling line, the threshold is reached far earlier than the operator expects.
Why B8M is not immune even though it is "316 with moly"
The 2 to 3 percent molybdenum addition in S31600 raises the Pitting Resistance Equivalent Number (PREN) above plain S30400. PREN is calculated as PREN = Cr + 3.3 × Mo + 16 × N. Using the A193 Table 1 composition limits for B8M with no deliberate nitrogen addition, the PREN range is approximately 22.6 at the low end of the spec to 27.9 at the high end. That is good enough to delay pit initiation in mildly chloride-laden water but it is well short of the PREN values needed to resist active Cl-SCC in hot service, where 40+ is the conventional benchmark for essentially immune.
The other half of the story is microstructure. B8M is a single-phase austenitic stainless. Austenite is the most chloride-SCC-susceptible of the common stainless families because crack tips can run continuously through the matrix without being arrested by ferrite islands. Duplex grades break this path with their 50/50 austenite-ferrite structure, and that is the single biggest reason a duplex bolt outperforms B8M in hot brine even when the bulk chemistry looks comparable.
PREN comparison across A193 austenitic and duplex bolting grades
| A193 grade | UNS | Cr (%) | Mo (%) | N (%) | PREN (low to high) | Cl-SCC class |
|---|---|---|---|---|---|---|
| B8 (304 SS) | S30400 | 18.0-20.0 | - | - | 18.0 to 20.0 | Susceptible |
| B8M (316 SS) | S31600 | 16.0-18.0 | 2.00-3.00 | - | 22.6 to 27.9 | Susceptible above threshold |
| B8MN (316N) | S31651 | 16.0-18.0 | 2.00-3.00 | 0.10-0.16 | 24.2 to 30.5 | Susceptible above threshold |
| B8MLCuN (6Mo super-austenitic) | S31254 | 19.5-20.5 | 6.0-6.5 | 0.18-0.25 | 42.2 to 45.9 | Essentially immune |
| B7 reference (low-alloy CrMo) | K14072 | 0.80-1.10 | 0.15-0.25 | - | n/a | Hydrogen-cracking risk separate |
Compositions per ASTM A193/A193M Table 1. PREN computed using PREN = Cr + 3.3 Mo + 16 N with low and high limits of each element band; PREN is a screening index, not a service guarantee.
Mechanical properties: why strain-hardened Class 2 B8M needs special handling
ASTM A193 supplies B8M in two distinct property classes. Class 1 (and the closely related 1A and 1D) is delivered solution-treated and ships at 75 ksi UTS / 30 ksi YS. Class 2 is strain-hardened to 110 ksi UTS / 95 ksi YS in the 3/4 inch and smaller diameter band, and to 95 ksi (B8M2) or 85 ksi (B8M3) in larger sizes. The strain hardening is what makes Class 2 B8M competitive on preload with B7, but it also raises the locked-in tensile stress on the surface and reduces the margin against SCC. In any chloride-borderline service the conservative call is to specify Class 1, accept the lower preload, and use a larger bolt diameter to make up the clamp force.
For very large diameters where Class 1 is preferred for SCC reasons and Class 2 is no longer available (above M20), the engineer is effectively forced into Class 1 anyway. Above M48 the spec does not list strain-hardened B8M at all.
See B8M Class 2 bolting for the strain-hardened product and B8M UNS S31600 material data for the full chemistry and mechanical property tables.
The chloride SCC service window for B8M bolting
The table below collates the chloride SCC initiation thresholds commonly applied in refinery and offshore practice. The numbers are corrosion-engineering rules-of-thumb drawn from ASM Handbook Volume 13 and from operator standards such as NORSOK M-001; they are not contractual guarantees. They are useful for screening a service against a candidate grade.
Chloride SCC initiation threshold by alloy class (austenitic and duplex bolting)
| Bolting class | Representative A193 grade | Approx. SCC initiation temperature | Practical chloride limit | Typical service note |
|---|---|---|---|---|
| 304 austenitic | B8 | ~50 °C | ~50 ppm | Not recommended for chloride-bearing duty |
| 316 austenitic | B8M | ~60 °C | ~100 ppm | Limit to non-insulated or low-chloride service |
| 316N nitrogen-strengthened austenitic | B8MN | ~65 °C | ~150 ppm | Marginal upgrade over B8M |
| 22Cr duplex | F51 / S31803 stud | ~110 °C | Tens of thousands ppm | Standard offshore upgrade above B8M |
| 25Cr super-duplex | F55 / S32760 stud | ~150 °C | Saturated brine | Subsea and topsides hot brine service |
| 6Mo super-austenitic | B8MLCuN / S31254 | Essentially immune at typical service temps | Saturated brine | Chloride-injection and seawater handling |
| Nickel alloy 625 / 718 | UNS N06625 / N07718 | No practical SCC limit in oxidising chloride | Saturated brine | Subsea wellhead, splash zone, sour + chloride combined |
Thresholds are screening values from corrosion-engineering practice (ASM Handbook Vol. 13, NORSOK M-001 selection guidance). Always confirm the specific service with a qualified materials engineer before issuing the spec.
Three duties where B8M is the wrong call
The first duty where B8M routinely fails is direct or splash-zone seawater. Seawater contains roughly 19,000 ppm chloride, two orders of magnitude above the B8M practical limit, and surface temperatures in the splash zone in tropical climates regularly run above the 60 °C threshold under solar heating. Studies dating back to the 1970s show 316 bolting in subsea or splash-zone duty pitting and then SCC-cracking within a few years. The standard offshore upgrade for this duty is 25Cr super-duplex or solid nickel alloy 718 / 725.
The second duty is corrosion under insulation. CUI on an austenitic stainless flange is classically caused by rainwater leaching chlorides out of the insulation, concentrating them at the metal surface as the water evaporates, and then re-wetting. The surface chloride concentration on the bolt can climb to thousands of ppm even though the bulk atmosphere is benign. Combined with operating temperatures of 80 to 150 °C, B8M will crack. The fixes are either to upgrade the bolting to duplex or to specify chloride-free insulation per ASTM C795.
The third duty is hot produced water and chloride-injection lines on upstream facilities. Here the chloride is high and the temperature is high. B8M is regularly retired from this service after one or two cracking events; the field-proven replacement is 22Cr duplex F51 studs for moderate temperature and super-duplex F55 for hotter duty.
Sour service overlap: how NACE MR0175 / ISO 15156 changes the picture
Chloride SCC and sulfide stress cracking are different mechanisms, but in real sour-and-chloride upstream service they appear together and the controlling document is NACE MR0175 / ISO 15156. B8M is listed in ISO 15156-3 with restrictions on H2S partial pressure, temperature, and chloride. In practice the MR0175 envelope for B8M is narrower than the atmospheric chloride SCC envelope, because adding even small partial pressures of H2S accelerates initiation of both SSC and Cl-SCC. Specifying B8M for sour duty requires a service-specific check against the alloy table and the temperature, pH2S, and chloride limits in MR0175.
Specifying B8M correctly: a four-line checklist
- Confirm the bulk service chloride concentration and the maximum metal temperature (including any insulation skin temperature). If either crosses the 100 ppm / 60 °C threshold, do not specify B8M; move up to duplex or 6Mo.
- For borderline service that still calls for B8M, specify Class 1 (solution-treated) rather than Class 2 (strain-hardened). The lower locked-in tensile stress materially extends the SCC initiation time.
- Pair B8M studs with A194 Grade 8M nuts (matched 316 chemistry). Do not mix B8M studs with carbon-steel nuts in chloride duty; galvanic and crevice effects accelerate pitting on the nut bearing face.
- Order with full 3.1 material certification per EN 10204, with explicit reporting of Mo and N content so the actual PREN can be calculated against the spec minimum.
For a side-by-side cost and service comparison of B8M against the plain-austenitic B8 option, see B8M vs B8 comparison. For the parent product family overview, return to the B8M home page.
References
- ASTM A193/A193M, Standard Specification for Alloy-Steel and Stainless Steel Bolting for High Temperature or High Pressure Service.
- NACE MR0175 / ISO 15156, Petroleum and natural gas industries, Materials for use in H2S-containing environments in oil and gas production, Parts 1, 2, and 3.
- ASM Handbook Volume 13, Corrosion: Fundamentals, Testing, and Protection (general SCC threshold guidance for austenitic stainless steels).
- ASTM C795, Standard Specification for Thermal Insulation for Use in Contact with Austenitic Stainless Steel.
- NORSOK M-001, Materials selection.
Related references
Compliance route: NACE MR0175 / ISO 15156-3 sour service. Material backstop: UNS S31600 datasheet. Sister alloy: B8M vs B8 comparison. Heat-treatment classes: Class 2 strain-hardened, Class 1 solution-annealed. Companion nut: A194 Grade 8M heavy hex. Spec hub: A193 B8M overview.
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