ASTM A193 Suffixes Explained: A, M, L, N, Cu and X in Bolting Materials

What Is ASTM A193?

ASTM A193 / A193M is a material specification covering alloy steel and stainless steel bolting materials intended for high-temperature or high-pressure service.

Typical applications include:

• Pressure vessel flanges
• Process piping bolting
• Valves and fittings
• Heat exchangers
• Oil & gas upstream and downstream equipment

ASTM A193 is usually used together with:

• ASTM A194 – nuts for high-pressure bolting
• ASME B16.5 / B16.47 – flanges
• ASME B31.3 – process piping
• NACE MR0175 / ISO 15156 – sour service requirements

Importantly, ASTM A193 applies only to bolting materials, not plates, pipes, or forgings.

Why ASTM A193 Suffixes Matter

In ASTM A193, suffixes are part of the grade designation, not optional descriptors.

For example:

• B7 ≠ B7M
• B8 ≠ B8A
• B8M ≠ B8MLN

Ignoring suffixes can lead to:

• Incorrect hardness levels
• Stress corrosion cracking (SCC) or sulfide stress cracking (SSC)
• Non-compliance with NACE requirements
• Project rejections or field failures

From a specification and procurement standpoint, suffixes must always be explicitly stated on:

• Data sheets
• Purchase orders
• Material requisitions
• Inspection and Test Plans (ITP)

Explanation of ASTM A193 Letter Suffixes

Suffix A — Cold Worked

Meaning
The suffix A indicates that the bolt or stud is produced using cold working processes, such as cold drawing, cold heading, or cold thread rolling.

Engineering significance

• Mechanical strength is achieved primarily through work hardening
• Commonly applied to austenitic stainless steel grades such as B8A, B8MA, B8TA
• Improves yield strength and tensile strength without heat treatment

Engineering caution

• Cold working increases hardness
• Excessive cold work may reduce ductility
• Not always suitable for high-temperature or severe stress-corrosion environments

Typical example

• B8A (cold-worked 304 stainless steel bolt)

Suffix M — Maximum Hardness Controlled

Meaning
The suffix M indicates that the material is subject to maximum hardness limits, typically required for sour service (H₂S environments).

Engineering significance

• Essential for compliance with NACE MR0175 / ISO 15156
• Achieved through controlled heat treatment and tempering
• Prevents sulfide stress cracking (SSC)

Typical application

• B7M instead of B7 in sour service

Typical hardness limit

• B7M: maximum 22 HRC (industry standard)

Critical reminder

• B7 and B7M have similar chemistry but very different service suitability
• Substituting B7 for B7M in sour service is a serious specification violation

Suffix L — Low Carbon

Meaning
The suffix L indicates low carbon content, typically C ≤ 0.03%.

Engineering significance

• Reduces susceptibility to intergranular corrosion
• Improves performance in welded or elevated-temperature environments
• Common in stainless steel bolting exposed to thermal cycles

Typical grades

• B8L, B8ML, B8LN, B8MLN

Common misconception

• Low carbon does not mean lower strength when nitrogen is also alloyed

Suffix N — Nitrogen Alloyed

Meaning
The suffix N indicates that nitrogen is intentionally added as an alloying element.

Engineering significance

• Nitrogen provides solid-solution strengthening
• Improves resistance to pitting and crevice corrosion
• Compensates for strength loss caused by low carbon

Typical applications

• 304N, 316N bolting
• High-strength stainless steel fasteners
• Super austenitic stainless steels

Engineering benefit

• Higher strength without increasing carbon
• Improved corrosion resistance in chloride environments

Suffix Cu — Copper Alloyed

Meaning
The suffix Cu indicates that copper is intentionally added to the steel.

Engineering significance

• Improves resistance to acidic corrosion
• Enhances performance in seawater and high-chloride environments
• Often used in offshore and marine applications

Typical grade

• B8MLCuN (UNS S31254 / commonly known as 254 SMO)

Key takeaway

• Copper is a functional alloying element, not an impurity

Suffix X — Special Chemistry Control

Meaning
The suffix X indicates special or modified chemical composition requirements beyond the standard base grade.

Engineering significance

• Not a standard off-the-shelf grade
• Typically project-specific
• Often associated with enhanced corrosion resistance or mechanical performance

Critical requirement

• Must be verified strictly against the Material Test Report (MTR)
• Cannot be accepted based on grade name alone

Engineering risk

• “X” grades require closer inspection and documentation review

Common ASTM A193 Suffix Mistakes

1. Omitting suffixes in purchase orders
   Leads to uncontrolled substitutions by suppliers.

2. Assuming chemical similarity equals equivalence
   Same chemistry does not guarantee same hardness or processing.

3. Using B7 instead of B7M in sour service
   One of the most common and dangerous mistakes.

4. Accepting MTRs without suffix verification
   Grade designation must match exactly.

How to Specify ASTM A193 Grades Correctly

A correct material callout should always include:

• ASTM specification
• Grade
• Suffix (if applicable)
• Heat treatment condition (if required)

Correct example

Stud Bolt: ASTM A193 Grade B7M, quenched and tempered, maximum hardness 22 HRC

Incorrect example

Stud Bolt: ASTM A193 Grade B7

The second example leaves too much room for misinterpretation and non-compliance.

Conclusion

ASTM A193 suffixes are critical engineering requirements, not optional labels. They define how a bolting material is processed, what chemical limits apply, how hardness is controlled, and where the material can be safely used.

Understanding suffixes such as A, M, L, N, Cu, and X allows engineers and procurement teams to:

• Avoid costly specification errors
• Ensure compliance with NACE and ASME codes
• Improve long-term equipment reliability
• Reduce corrosion and cracking risks

When specifying ASTM A193 bolting materials, always write the full grade designation—including suffixes. In bolting applications, details matter.