Heat Treatment of Valve Castings and Weldments

Why Castings Cannot Skip Heat Treatment

An as-cast valve body is metallurgically unfinished. Solidification in the mold produces coarse, uneven grain structures, segregated alloying elements, and internal stresses locked in by uneven cooling through thick and thin sections. Left in that condition, the casting will deliver erratic mechanical properties, poor toughness at low temperature, and a shortened fatigue life.

Heat treatment resets the microstructure. Depending on the material and the service, the goal is grain refinement, softening, stress relief, or the dissolution of unwanted phases. For pressure-retaining valve components, the applicable standards — ASME B16.34 for material requirements, ASTM A216 and A351 for carbon and stainless castings — do not merely permit heat treatment; they define the cycles that make the material qualify as the grade on the certificate.

What Each Cycle Is For

The main treatments you will encounter on valve components serve distinct purposes:

• Normalizing — heating above the upper critical temperature and cooling in still air. Refines coarse as-cast grain and homogenizes the structure.
• Full annealing — furnace cooling for maximum softening and machinability, at the cost of some strength.
• Tempering / stress relieving — subcritical heating to reduce residual stress and adjust toughness.
• Solution annealing — for austenitic and duplex stainless steels, dissolving carbides and secondary phases, followed by rapid cooling.
• Post-weld heat treatment (PWHT) — applied to weldments to temper the heat-affected zone and relieve welding stresses.

A body-bonnet casting in carbon steel typically receives normalizing plus tempering. An austenitic casting typically receives solution annealing. Neither step is decorative — skip it and the material on the EN 10204 certificate is not the material in the valve.

Material-Specific Cycles: WCB and CF8M

WCB: Normalizing and Tempering in Practice

ASTM A216 WCB is the workhorse carbon steel casting for Class 150 through Class 2500 gate valves, globe valves, and check valves. The standard requires normalizing or normalizing plus tempering (or annealing plus tempering for some configurations) above a specified minimum temperature.

Why temper after normalizing? Normalizing refines the grain and sets the strength, but the resulting structure can carry residual stress and can be tougher than needed in hardness terms for machining. Tempering at a controlled subcritical temperature relaxes those stresses and stabilizes the mechanical properties, particularly yield strength and Charpy behavior. For WCB destined for low-temperature or cyclic service, buyers often specify impact testing in addition to the standard requirements — and impact results are acutely sensitive to whether the normalizing and tempering cycle was performed correctly.

CF8M, Solution Annealing, and Sensitization

CF8M — the cast equivalent of 316 stainless — presents a different problem. During solidification and slow cooling, chromium carbides precipitate at grain boundaries. That chromium-depleted zone is what we call sensitization, and it is the open door to intergranular corrosion in chloride-bearing and acidic environments.

Solution annealing dissolves those carbides: the casting is heated to a high temperature, held long enough for carbon to go back into solution, then cooled rapidly — usually by water quench — through the carbide precipitation range. Done correctly, the result is a carbide-free, fully austenitic (with a controlled ferrite content) structure resistant to intergranular attack.

The failure mode to watch for is a partly sensitized condition: the casting was annealed, but cooled too slowly, or was later heated by welding without a subsequent re-anneal. The certificate shows solution annealing; the microstructure says otherwise. This is exactly the situation that intergranular corrosion testing, discussed in most serious valve specifications, is designed to catch.

PWHT: When and How

Post-weld heat treatment applies wherever code, specification, or material thickness requires it. Typical triggers on valve assemblies:

• Wall thickness above the code threshold for the material group.
• Client specification for sour service, fatigue service, or low-temperature service.
• Hardness limits such as those in NACE MR0175 / ISO 15156 for wet H2S service.
• Major weld repairs on castings, which often mandate re-tempering of the repair area.

Temperature control is the discipline that separates adequate PWHT from box-ticking. The holding temperature band, soak time proportional to thickness, controlled heating and cooling rates, and thermocouple placement all determine whether the heat-affected zone actually tempers. A PWHT run with the workpiece lagging the chart thermocouple — a classic shop-floor error — can leave the weld hard and the certificate rosy.

Verifying the Result: Coupons and Charts

Test Coupons and Mechanical Property Verification

Heat treatment is verified by destruction — of a coupon, not the valve. Castings are poured with attached test coupons (keel blocks or comparable), which travel through the heat treatment furnace with the production castings. The coupon is then machined into tensile and, where required, impact specimens and tested against the standard's minimum values.

This arrangement only means something if the coupon genuinely experienced the same thermal cycle as the casting: same charge, same position in the furnace, same cycle. Responsible foundries document charge composition and furnace position for each run. Buyers reviewing an ITP should treat "coupon heat treated with production castings" as a line item, not an assumption.

Reading the Temperature Charts

Furnace charts are the single most informative record in the heat treatment dossier. A meaningful review checks:

• Hold temperature against the specified band for the grade, not just "was hot."
• Soak duration against thickness-based requirements.
• Heating and cooling rates, particularly for PWHT and quench transitions.
• Chart calibration status and furnace survey records (temperature uniformity surveys).
• Consistency between the chart, the heat number log, and the test coupon results.

Charts that show flat-topped holds with clean ramps indicate a controlled furnace. Jagged traces, overshoot, or a hold that conveniently starts and ends exactly at the minimum soak time deserve a second look.

Weld Repairs and Re-Heat Treatment

Castings get weld repaired; that is normal foundry practice and is governed by qualification requirements on the repair procedure, the welders, and the NDT that proves the repair is sound. What is sometimes overlooked is that a substantial repair re-introduces a heat-affected zone into a material that was previously heat treated to final condition.

For carbon steel castings, the repair is usually followed by local or furnace re-tempering, then re-NDT and, where required, re-testing of a companion coupon. For austenitic castings, a significant repair may require re-solution annealing — a decision that affects distortion risk and must be planned, not improvised. The valve datasheet and the applicable casting specification define which repairs trigger re-treatment; the ITP should state it explicitly so the manufacturer's practice and the buyer's expectation match before the arc strikes.

When Heat Treatment Goes Wrong

The consequences of an incorrect cycle are rarely subtle:

• Quench cracking in hardenable grades cooled too aggressively, or in castings with abrupt section changes.
• Mechanical properties below the minimum — low yield from an under-temperature normalize, low toughness from a missed temper.
• Residual sensitization in CF8M, discovered months later as intergranular corrosion at a weld or flange face.
• Excessive hardness in weldments skipped through PWHT, failing NACE hardness surveys.
• Distortion from uncontrolled heating, quietly ruining seat flatness that dimensional inspection will then have to reject.

Every one of these is cheaper to prevent by procedure review and chart scrutiny than to discover in the field.

Acceptance Basis and What to Ask For

Acceptance rests on the interlocking documents: the heat treatment record and charts, the coupon test results per ASTM A216 / A351 / A182 as applicable, hardness reports where specified, and NDT clearances after any repair. For a gate valve order in carbon or stainless steel, ask the manufacturer to define the thermal cycles in the ITP itself, with the chart review assigned as a documented review point. It is also worth checking how the valve manufacturer structures its material and testing documentation for each pressure class before the order is placed.

A manufacturer that produces and supplies the full range of industrial valves directly — ball valves, control valves, gate valves, globe valves, check valves, strainers, knife gate valves, plug valves, and butterfly valves — will typically have an engineering team that specifies heat treatment cycles, coupon testing, and documentation scope as part of every engineered quotation. Send your Valve Schedule or operating parameters to the manufacturer's applications team, and it can confirm the material and heat treatment plan that fits your service conditions before the first casting is ordered.