Does every ASME Section IX WPS require Charpy impact testing?

No. Charpy V-notch (CVN) impact testing under QW-170 is a supplementary essential variable — it is only required when the construction code (ASME VIII Div 1, B31.3, etc.) or the engineering specification specifically requires impact-tested weld procedures. When the construction code does not invoke impact testing, QW-170 does not apply and no CVN tests are needed on the PQR.

What is the difference between a supplementary essential variable and an essential variable in ASME IX?

Essential variables (QW-250) are always required — changing one requires requalification. Supplementary essential variables (also listed in QW-250 for each process) only become essential when the construction code requires notch-toughness testing. If CVN testing is not required by the applicable code, supplementary essential variables can be changed without requalification.

Where in the PQR must CVN impact test results be recorded?

Impact test results are recorded in Section QW-170 of the PQR (ASME QW-483 form). Record the specimen orientation (weld metal or HAZ), test temperature, individual absorbed energy values (ft-lbf), lateral expansion (mils), and the specimen size. All three specimen values and the average must be shown — not just the average.

Can AWS D1.1 CVN-tested filler metal certificates substitute for ASME IX PQR impact test results?

No. ASME Section IX requires that CVN impact specimens be taken from the actual procedure qualification test weld (the PQR coupon). Filler metal classification test data or manufacturer certificates cannot substitute for testing of the deposited weld metal in your specific joint configuration and heat input range.

ASME Section IX Impact Test Requirements: QW-170

ASME Section IX QW-170 governs Charpy V-notch impact testing for weld procedures. What triggers the requirement, test acceptance criteria, and essential variables.

Charpy V-notch (CVN) impact toughness testing for welded joints under ASME Section IX is one of the most frequently misunderstood requirements in pressure vessel and piping fabrication. Shops new to low-temperature service or post-2019 ASME Codes sometimes discover mid-project that their existing PQRs lack impact data — and that qualifying new procedures with CVN testing adds lead time and cost they did not budget.

Understanding when QW-170 applies, what it requires, and which variables become essential under its trigger conditions protects fabricators from last-minute requalification scrambles.

What QW-170 Is and When It Applies

ASME Section IX QW-170 is the impact test portion of the procedure qualification record framework. It defines:

The specimen types and orientations allowed
The test temperature requirements
The acceptance criteria for absorbed energy (ft-lbf) and lateral expansion (mils)
The essential variable changes that invalidate a CVN-qualified PQR

Critically, QW-170 does not self-trigger. The provision only becomes mandatory when the construction code — ASME Boiler and Pressure Vessel Code (BPVC) Section VIII Division 1, Division 2, Section I, or the applicable piping code (B31.1, B31.3) — requires notch-toughness testing for the material or service conditions.

Common triggers:

Low-temperature service: ASME VIII Div 1 UCS-66 (for carbon and low-alloy steels) requires impact testing when operating temperature falls below the curve-established minimum for the material and thickness. Many carbon steel vessels operating below −20°F (−29°C) fall into this zone.

High-strength low-alloy (HSLA) or quenched-and-tempered steels: Certain P-No. 1 Gr. 2 and P-No. 4 steels at heavier thicknesses require impact-tested procedures under the construction code.

User-specified requirements: The engineering specification or purchase order may require CVN testing even when the construction code does not — particularly for ASME nuclear or defense applications, or for owner specifications that impose more conservative toughness requirements.

If none of these triggers apply, QW-170 is informative only. Do not perform impact testing and add cost unnecessarily — but document why the requirement does not apply so the reviewer understands the deliberate decision.

Supplementary Essential Variables Activated by QW-170

When CVN testing is required, supplementary essential variables from QW-250 (the process-specific essential variable tables) become binding. A change to these variables requires a new PQR with impact testing.

For SMAW (QW-253), supplementary essential variables activated when CVN applies include:

Change in base metal P-Number/Group Number (not just P-Number as with standard essential variables — the Group Number within P-No. 1 matters for CVN purposes)
Change in filler metal F-Number
Change in A-Number (weld metal chemical composition group)
Increase in heat input beyond the qualified range
Decrease in preheat temperature below the qualified minimum
Change in PWHT (post-weld heat treatment) requirements — either adding, removing, or changing the temperature band
Increase in base metal thickness above the qualified range

The pattern: all the variables that affect heat input, thermal cycle, and weld metal composition become essential when you add CVN. This means a PQR qualified without CVN testing cannot simply have specimens cut and tested after the fact — the welding variables must be tracked at the time of qualification to bound what the CVN results represent.

For GMAW, FCAW, SAW, and GTAW, review the applicable QW-25x table for the process. The supplementary essential variable lists differ by process. SAW in particular has flux-related supplementary essential variables that do not appear in the SMAW list.

Test Specimens: Orientation, Location, and Number

QW-170.2 specifies the test specimens. Impact specimens may come from:

Weld metal (WM) specimens: Taken from the weld deposit, oriented so the notch passes through the weld centerline or near-centerline fusion zone. These assess the toughness of the deposited weld metal itself — most sensitive to filler metal selection, heat input, and PWHT.

Heat-affected zone (HAZ) specimens: Taken from the base metal immediately adjacent to the fusion line, oriented so the notch intercepts the HAZ. These assess the toughness of the base metal after being thermally cycled by the welding process — most sensitive to preheat, heat input, and base metal chemistry.

The construction code specifies which orientation is required. Many ASME VIII applications require both weld metal and HAZ specimens. Some P-No. 1 Gr. 1 applications at moderately low temperatures require only weld metal. Read the specific UCS or UHA reference in the construction code — do not assume.

Three specimens are required per test location and temperature per QW-170.2. Results must report all three individual values, the average, and the specimen size (full-size 10 mm × 10 mm preferred; sub-size 7.5 mm × 10 mm or 5 mm × 10 mm when material thickness limits it). Do not discard low individual values — the acceptance criteria assess both the average and the individual minimum.

Acceptance Criteria

QW-171.1 provides the minimum acceptable CVN values for P-No. 1 (carbon steel) materials:

Weld metal: 20 ft-lbf (27 J) average; no individual specimen below 15 ft-lbf (20 J)
HAZ: 20 ft-lbf (27 J) average; no individual specimen below 15 ft-lbf (20 J)

These are Section IX minimum values. The construction code or engineering specification may require higher values — for example, cryogenic service applications often specify 35 ft-lbf or higher at −150°F. The specification governs when more stringent than QW-171.1.

For other P-Numbers (stainless, nickel alloys, duplex), consult the applicable QW-171 paragraph for that material grouping. Austenitic stainless steels (P-No. 8) are generally exempt from CVN requirements under most construction codes because of inherently high toughness — but not always; confirm for your application.

Lateral expansion is an alternative or supplemental acceptance criterion. QW-171.1 accepts 15 mils (0.38 mm) lateral expansion as an alternative to the absorbed energy minimum. Some construction codes require lateral expansion measurement in addition to energy absorption, not as an alternative. Record both even when only one is the formal acceptance criterion — the extra data costs nothing and prevents re-testing if the code interpretation changes during review.

Test Temperature

CVN specimens are tested at the temperature specified by the construction code for the material and service conditions. For Section VIII Div 1 carbon steel applications under UCS-66, the test temperature is typically:

The minimum design metal temperature (MDMT) of the vessel, or
At a temperature below the MDMT per the specified margin, or
The temperature on the exemption curve for the material

The PQR must record the actual test temperature. The WPS should reference the minimum test temperature from the PQR as a bound — a change in required service temperature that drops below the qualified PQR test temperature is a supplementary essential variable change requiring requalification or additional testing.

What the PQR Must Document for QW-170

When CVN testing is performed, the PQR (ASME Form QW-483) must include:

Section QW-170 filled out with all test results
Number of specimens tested at each location and temperature
Individual and average absorbed energy (ft-lbf and J)
Individual and average lateral expansion (mils and mm)
Specimen size if sub-size was used (full-size assumed if not stated)
Test temperature
Acceptance per QW-171 or the specified criteria

The welding parameters recorded in the PQR must bound the heat input range used during testing. Calculate arc energy per the AWS D1.1 formula (or ASME equivalent) for each pass position and record the range. Future production welds must fall within this range for the CVN results to remain valid.

If PWHT was performed on the test coupon, document the actual PWHT cycle (temperature ramp rate, hold temperature, hold time, cooling rate). A PWHT change is a supplementary essential variable when CVN applies.

Comparing ASME IX to AWS D1.1 CVN Requirements

Engineers and CWIs who work under both codes sometimes confuse the frameworks. Key differences:

Trigger mechanism: AWS D1.1:2025 Table 6.8 (supplementary essential variables for CVN) activates when the contract documents specify notch-toughness testing — essentially the same trigger as ASME QW-170. But the specific essential variables listed differ between the two codes.

Filler metal certification: AWS D1.1 permits the use of filler metals with AWS-classified CVN properties (e.g., E7018-C3L for low-temperature service) without PQR impact testing in some circumstances. ASME Section IX does not offer this path — the actual deposited weld metal in the qualification test weld must be tested. This is a fundamental difference in philosophy.

Acceptance values: AWS D1.1:2025 Table 6.8 references 20 ft-lbf minimum unless otherwise specified. ASME QW-171.1 also uses 20 ft-lbf minimum for P-No. 1. The values coincide at this baseline, but construction code requirements above this floor diverge by application.

For shops that qualify procedures under both codes, maintain separate PQR documentation systems. An ASME-qualified PQR cannot substitute for an AWS D1.1 PQR on a structural application (or vice versa) even if the test parameters were identical.

For related reading, see ASME Section IX P-Numbers and F-Numbers explained and ASME Section IX three variable types. To manage your WPS and PQR documentation — including CVN test records — in one place, explore the WPS platform at /pricing.

Rule library based on AWS D1.1:2025; verify against your governing edition. ASME references are based on ASME BPVC Section IX — consult your applicable edition and addenda. This article is educational; always verify requirements with the construction code and engineering specification governing your project.