Simulation Heat Treatment Requirements as per ASME Section VIII

Simulation heat treatment (SHT) is one of the most frequently misunderstood yet critically important requirements in pressure vessel fabrication. When a dish head or shell plate undergoes thermal processing — normalizing, stress relief, or both — the mechanical and impact properties of the material can change significantly from the original mill-supplied condition. ASME Section VIII Division 1 and Division 2 therefore require that representative test coupons be heat treated under identical conditions to the production component, and that mechanical testing be performed to verify the material properties remain compliant. Getting this requirement wrong can result in rejected welder qualification records, failed hydrotest, or, more seriously, the release of untested material into service.

This guide provides a comprehensive, clause-by-clause breakdown of simulation heat treatment requirements under ASME Section VIII, covering dish head plates received without SHT from the mill, shell plate test coupon requirements, and the interpretation of Material Test Certificates (MTC) where normalizing and stress relief are already confirmed. Each scenario is explained with the specific code clause, required tests, and practical engineering notes to help fabricators and inspection personnel make compliant, defensible decisions on the shop floor.

Whether you are a CWI, CSWIP inspector, ASME-certified fabricator, or pressure vessel engineer, this article will equip you with a complete understanding of when SHT is mandatory, when it can be waived, and exactly what documentation is required at each stage. For related topics, see our comprehensive guide to heat treatment in fabrication and the P91 material welding requirements.

1. What Is Simulation Heat Treatment and Why Is It Required?

Steel does not behave as a static material. Every time a piece of steel is heated above a certain temperature and then cooled, its microstructure changes. Normalizing refines the grain structure; stress relief reduces internal residual stresses without fully recrystallizing the material; combinations of both are used to balance strength, toughness, and ductility in finished pressure vessel components. These microstructural changes affect tensile strength, yield strength, elongation, and especially notch toughness (Charpy impact energy).

The mill that produces the steel plate performs a defined heat treatment and then tests a coupon from the same heat to generate the Material Test Certificate. However, the fabricator may expose the same plate to further thermal cycles — forming, welding preheat, PWHT, normalization in the shop — all of which add to the cumulative thermal exposure. If these additional cycles are severe enough, they can push the material properties outside the specification limits, even if the original MTC shows compliant results.

Simulation heat treatment is the code’s mechanism to verify that the material will still be compliant after all the thermal cycles it will see during fabrication. By testing a coupon that has been subjected to the same cumulative thermal history as the production component, the fabricator demonstrates, before committing to production, that the material will remain fit for service.

1.1 Test Coupon Dimensions and Sampling

The standard test coupon for simulation heat treatment under ASME Section VIII is approximately 100 mm (4 in) wide x 300 mm (12 in) long x full plate thickness. The thickness dimension is the critical one: because thermal gradients through thick plates differ from thin ones, the coupon must replicate the production plate thickness to ensure the heat treatment cycle produces a representative microstructure. The coupon must be cut from the same heat (cast) of material as the production plate, and from the same rolling orientation where impact test direction is relevant.

2. Dish Head Plate Received Without SHT from the Manufacturer

The most common scenario requiring fabricator-performed simulation heat treatment arises when a dish head plate is supplied by the mill without any prior simulation heat treatment. In this case, the MTC will show the mill’s normalizing or stress relief data, but no simulation test coupon results. Before the fabricator can form and use the plate, they must run their own SHT coupon under conditions that mirror the full thermal cycle the finished dish head will experience.

2.1 Applicable Code Clauses

The governing requirements are found in UCS-85(c) for Division 1 and clause 3-10.2 for Division 2. These clauses state that when heat treatment is required during fabrication and the test coupon provided by the mill has not been subjected to the equivalent thermal cycle, the fabricator must perform simulation heat treatment and testing before using the material.

2.2 The Complete Thermal Cycle for a Dish Head

A dish head fabricated from carbon or low-alloy steel plate will typically experience the following thermal sequence during fabrication:

The simulation heat treatment test coupon must replicate all three stages: forming temperature, normalizing, and stress relief. If the coupon is only subjected to normalizing without the forming temperature cycle, it will not accurately represent the thermal history of the production head, and the test results will be unconservative.

2.3 Required Tests After Simulation Heat Treatment

3. Shell Plate — Simulation Test Coupon with Stress Relief Only

Shell plates for pressure vessels present a somewhat different situation compared to dish heads. Shell plates are typically not hot-formed and may not require normalizing. However, they will still undergo stress relief (PWHT) after welding, which can alter their mechanical properties — particularly impact toughness. The code addresses this through specific provisions that differentiate between materials and stress relief temperatures.

3.1 The Low Transformation Temperature (LTT) Exemption

UCS-85(f) provides a key exemption for P-No. 1 materials: if the stress relief temperature is below the Low Transformation Temperature (LTT) of the steel, no simulation heat treatment is required. The rationale is that below the LTT, the steel’s microstructure is not significantly altered by the thermal cycle, so the mill-produced mechanical properties remain essentially unchanged.

The practical implication is that most carbon steel shell plates, stress relieved at the code-minimum PWHT temperature, qualify for the LTT exemption. However, the exemption does not automatically eliminate the need for a simulated SR test coupon when Charpy impact testing is required. This is because even a modest SR cycle can reduce notch toughness in certain steels, particularly normalized-and-tempered grades or fine-grained steels, and the impact test must be performed on material in its final condition.

3.2 When Shell Plate SHT Is Required

3.3 Required Tests for Shell Plate SR Simulation Coupon

When a simulation SR test coupon is required for a shell plate, the minimum test requirements are:

1. Charpy V-Notch Impact Test — at the design minimum temperature specified by the code or project specification, ensuring the simulated SR condition does not cause embrittlement.
2. Tensile Test — to verify that SR has not over-softened the material below the specified minimum yield or UTS values.
3. Hardness Test — where required by the material standard or project specification, particularly relevant for sour service applications. See our guide on sour service material requirements.

4. Dish Head — MTC Showing Normalizing + Stress Relief Already Performed

A common practical question arises when the Material Test Certificate for a dish head already shows that both normalizing and stress relief were performed by the mill: is further stress relief by the fabricator mandatory? Can the existing mill SHT data be used, or must the fabricator run a new coupon?

4.1 Interpreting the MTC — What Counts as Acceptable SHT Evidence?

For the mill-performed heat treatment data to satisfy the fabricator’s SHT requirement, the following conditions must be met:

1. The MTC must explicitly confirm that a simulation heat treatment test coupon was subjected to the same thermal cycle (normalizing + SR) as the production plate, not merely that the plate itself was heat treated.
2. The heat treatment parameters recorded on the MTC — temperature, holding time per mm of thickness, heating and cooling rate — must match or bound the parameters the fabricator will use during fabrication.
3. The mechanical test results (tensile, impact, hardness as applicable) must be recorded against the simulation coupon, not just the plate in its as-delivered condition.
4. No additional thermal cycles are introduced during fabrication that exceed the parameters already documented on the MTC.

4.2 The UCS-85(f) Position When MTC Confirms Normalizing + SR

If the MTC confirms Normalizing + SR and the test coupon has been similarly treated, and the fabrication process does not introduce any additional thermal cycles above 480°C that are not already accounted for in the MTC, then per UCS-85(f), no further simulation heat treatment is required by the fabricator. This is the most common outcome for dish heads supplied by major international mills that follow ASME procedures.

However, if the fabricator performs any of the following operations, a new SHT assessment is needed:

• Further hot forming that re-heats the material above the normalizing temperature
• Welding repairs on the formed head followed by PWHT at different parameters than the mill’s SR
• Additional stress relief required by the vessel design code that uses a higher temperature or longer hold time than the mill SR recorded on the MTC

5. Cumulative Heat Treatment Concept — Practical Implications

One of the most important principles underlying simulation heat treatment is the concept of cumulative thermal exposure. Each time a piece of steel is heated and cooled, the thermal energy drives diffusion, grain growth, carbide precipitation, and other microstructural changes. These effects accumulate across all the thermal cycles the material experiences from steel making through final fabrication. The simulation test coupon must capture this cumulative exposure to give meaningful test results.

5.1 Calculating Equivalent PWHT Hours

For stress relief cycles, the concept of equivalent hours at a reference temperature is used to combine multiple SR cycles with different temperatures and durations into a single equivalent thermal exposure. The Larson-Miller parameter (P) is commonly used for this purpose:

5.2 Multiple Weld Repairs and Simulation Coupon Relevance

In heavy-wall pressure vessel fabrication, it is not uncommon for shell courses to require one or more weld repairs after the initial PWHT. Each repair cycle followed by PWHT adds to the cumulative thermal exposure. If the total number of thermal cycles exceeds the number simulated on the test coupon, the coupon is no longer representative. In such cases:

• Prepare a new test coupon from the same plate heat before welding begins
• Subject it to the maximum expected cumulative PWHT exposure (accounting for the possibility of two full repair cycles)
• If real conditions exceed the simulated coupon, obtain a new plate coupon and re-test before releasing the component for service

6. Documentation Requirements and Quality Records

ASME Section VIII requires that all simulation heat treatment records — including heat treatment charts, test coupon identification, and mechanical test reports — be maintained as part of the pressure vessel data report package. These records are reviewed by the Authorized Inspector (AI) during code stamp certification and must be retained for the life of the vessel.

6.1 Minimum Documentation Package

7. Comparison: Division 1 vs Division 2 SHT Requirements

While Division 1 and Division 2 of ASME Section VIII share the same underlying principle for simulation heat treatment, there are important procedural differences that engineers and inspectors must be aware of when working across both divisions.

8. Common Non-Conformances and How to Avoid Them

Experience across ASME-stamped fabrication shops reveals several recurring non-conformances related to simulation heat treatment. Understanding these pitfalls helps quality engineers build more robust Inspection and Test Plans (ITPs).

8.1 Coupon Not From the Same Heat Number

Using a coupon from a different plate heat than the production material is a fundamental non-conformance. Even if the chemistry is nominally the same, heat-to-heat variation in cleanliness, grain size, and alloying balance means the test results are not representative. Always verify coupon and production plate heat numbers match before proceeding.

8.2 Simulation Does Not Include All Thermal Cycles

The most frequent error is a coupon that is only stress relieved when the production component will also be normalized. This typically happens when the forming contractor and the PWHT contractor work in sequence without coordination at the quality planning stage. Build the full thermal cycle map into the ITP at the outset.

8.3 MTC Heat Treatment Parameters Insufficient for Fabrication

If the mill MTC shows an SR hold time of 1 hour at 620°C but the fabricator will hold for 3 hours (due to a thicker weld), the mill SHT data is not bounding. A new coupon subjected to the longer hold time must be tested. Check the hold time calculation against plate thickness per the code requirements for PWHT.

8.4 Inadequate Documentation

A recurring finding during ASME audits is that furnace charts are missing or show excursions above the target temperature. All furnace charts must be reviewed and accepted before the simulation heat treatment is considered complete. For more on ASME inspection requirements and quiz-based preparation, see our ASME Section VIII Division 1 quiz.

9. Simulation Heat Treatment for Special Material Conditions

9.1 Impact-Tested Fine-Grained Steels (SA-516 Gr. 70N)

SA-516 Grade 70 in the normalized condition (SA-516-70N) is one of the most commonly used pressure vessel plate materials. When used in impact-tested service (typically below -20°C MDMT), the simulation heat treatment is almost always required because the Charpy impact requirements are demanding and SR is known to affect toughness. The simulation coupon for SA-516-70N must include normalizing followed by stress relief at the exact temperature and holding time specified in the vessel PWHT procedure.

9.2 P91 and Cr-Mo Alloy Steels

For creep-resistant alloy steels such as P91 (Grade 91) and other Cr-Mo materials, simulation heat treatment requirements are governed by different UCS clauses and the relevant material specification. The thermal sensitivity of P91, particularly the risk of over-tempering or sub-critical annealing, makes precise simulation heat treatment even more critical. See our dedicated article on P91 welding and heat treatment requirements for detailed guidance. For high-alloy and stainless steel applications including duplex grades, the simulation requirements differ significantly — refer to our duplex stainless steel welding guide.

9.3 Sour Service Carbon Steel

Carbon steel components intended for sour service applications (H2S environments per NACE MR0175 / ISO 15156) carry additional hardness restrictions that must be verified after heat treatment. Post-weld heat treatment for sour service must achieve hardness values below 22 HRC (typically ≤200 HBN for the base material HAZ) throughout the component. Simulation heat treatment test coupons for sour service vessels must include hardness testing as a mandatory requirement. See our sour service guide for detailed requirements.

10. Key Takeaways: Simulation Heat Treatment Decision Guide

Recommended Reference Books

Disclosure: WeldFabWorld participates in the Amazon Associates programme (StoreID: neha0fe8-21). If you purchase through these links, we may earn a small commission at no extra cost to you. This helps support free technical content on this site.

Frequently Asked Questions

Related Technical Articles