T92 High Alloy Pipe Welding: Preheat, Root Pass and Heat Treatment Control

Why T92 Joints Need a Dedicated Welding Procedure

T92 is a martensitic 9% chromium ferritic heat-resistant steel developed on the basis of T91. Its chemistry is rebalanced so that molybdenum is lowered to 0.30-0.60% while tungsten is raised to 1.50-2.00%, producing a W-Mo composite solid solution strengthening effect in which tungsten is the dominant element. Nitrogen is added for interstitial solid solution strengthening, vanadium and niobium combine with nitrogen to form fine carbonitride dispersions, and a trace of boron at 0.001-0.006% strengthens the prior austenite grain boundaries. The combination gives markedly higher creep rupture strength than Grade 91 at the same metal temperature.

Typical composition ranges for Grade 92 tube and pipe are listed below. They follow the requirements of the applicable product specification for this grade on both the ASTM and ASME side.

Element	Range (%)	Element	Range (%)
Carbon	0.07-0.13	Chromium	8.50-9.50
Manganese	0.30-0.60	Molybdenum	0.30-0.60
Silicon	0.50 max	Tungsten	1.50-2.00
Vanadium	0.15-0.25	Niobium	0.04-0.09
Nitrogen	0.030-0.070	Boron	0.001-0.006

Because of this balance of strength and steam oxidation resistance, T92 is specified for superheaters and reheaters in power plant boilers, for headers, and for main steam and hot reheat piping operating under extremely severe steam conditions. It is also used for pressure vessel shells and high temperature pressure parts of nuclear power plants. Every one of those components is completed by welding, and the weldment normally governs the creep life of the whole line, so the welding procedure is an engineering document rather than a workshop routine.

Preheating: Where a T92 Joint Is Won or Lost

Preheating begins at the centre of the joint and extends to each side by not less than three times the weld thickness and never less than 100 mm. A neutral flame is used and the torch is moved evenly and continuously; the flame must never be parked at one position, because local overheating produces soft, coarse patches that later become creep cavities.

Temperature is measured and recorded with a portable infrared thermometer. Preheat is controlled at 100-200 °C. Once the specified temperature has been reached the joint is held for at least 3 minutes before the arc is struck, so that the heat has soaked through the wall instead of sitting on the surface. The interpass temperature must never fall below the preheat temperature and must never exceed 250 °C.

Preheat band: not less than 3 x wall thickness and not less than 100 mm on each side.

Preheat range: 100-200 °C, measured and logged.

Soak at temperature: minimum 3 minutes before welding.

Interpass window: from the preheat temperature up to 250 °C maximum.

Root Pass and Internal Argon Purge

Tack welds are treated exactly like production welds: DC electrode negative, high frequency arc starting, and a tack length not exceeding 10 mm with a thickness not exceeding 3 mm. Tacks are placed where they do not obstruct the welder's line of sight, and each tack becomes part of the root pass, so full fusion is mandatory. If a tack shows a defect it is removed at once and repeated.

For gas tungsten arc welding, argon is supplied in advance and cut off with a delay. After the arc is extinguished the shielding gas continues to flow over the pool until no dark red colour remains, which is the moment at which the weld can be left unprotected. During root deposition argon is injected from the groove gap with an air needle and the flow is held at 8-10 L/min; aluminium foil taped across the pipe is peeled back progressively so that one section is welded at a time and the internal shield is never lost.

The torch angle is watched continuously so that the gas column covers the molten pool, and filler wire is fed at a steady angle without being allowed to disturb the shield. After each addition the hot wire tip stays inside the argon envelope; pulling it out into air oxidises the tip and drags oxide into the next pass. Root thickness is generally 2.4-3.0 mm. Penetration and groove wall must fuse cleanly, with no nodules or protruding wire ends, and the root may never be pushed through by leaning on the filler wire. The root surface is inspected before any filling pass is started.

Filler and Cover Pass Parameters

The layer immediately after the root is still deposited by gas tungsten arc welding, which prevents burn-through and protects the root from oxidation. Subsequent filler passes are made with 2.5 mm electrodes. The layer thickness is kept to 2.5 mm maximum, the oscillation width of a single bead to 10 mm maximum, and the arc energy to 22 kJ/cm maximum. Filler metals for Grade 92 are classified as ER90S-B9 wire for tungsten arc welding and E9015-B9 covered electrodes for shielded metal arc welding.

Pass	Consumable	Thickness / width limit	Arc energy
Root	ER90S-B9, internal argon purge	2.4-3.0 mm	low, controlled
Second layer	ER90S-B9, tungsten arc	2.5 mm max per layer	22 kJ/cm max
Filler passes	E9015-B9, 2.5 mm electrode	2.5 mm max, bead 10 mm max	22 kJ/cm max
Cover	E9015-B9	bead 2-3 mm thick, 8 mm wide max	22 kJ/cm max

Layer starts are staggered so that no two joints sit in the same plane, and each stop is filled before the arc is broken, otherwise crater cracks form. Oxide and coating left between layers are ground away; the weld is never hammered or struck with a tool to remove slag, because Grade 92 tolerates such impact poorly. The next layer is only started after the previous one has been confirmed free of defects.

Before the cover pass the groove is checked to see that the filling layer lies about 1 mm below the groove face. If the filler layer has risen above the face or is uneven, it is ground or repaired to keep the finished weld profile consistent. On a wide groove the cover is run in multiple stringer beads 2-3 mm thick and not more than 8 mm wide, with no deep valley between adjacent beads.

Pool temperature is the controlling variable at the cover stage: the dwell time at each side wall is roughly twice that at the centre of the bead. Where access is difficult, the difficult side is welded first so that it does not have to be reached over a finished contour.

Post-Weld Heat Treatment, Hardness and Inspection

As-welded Grade 92 weld metal and heat affected zone are hard and brittle, so post-weld heat treatment is mandatory before the joint enters service. In normal power plant practice the treatment temperature for 9Cr-2W weldments falls in the region of 730-770 °C, with the exact holding temperature, holding time and heating and cooling rates fixed by the design code and by the wall thickness of the component. The whole preheat band, and enough additional length to avoid harmful gradients, is covered by the heating elements.

After treatment the joint is checked for surface condition and, where specified, for hardness across the weld, heat affected zone and base metal to confirm that the hard as-welded structure has been tempered. Non-destructive examination is carried out on the root before filling and on the completed joint, and pressure parts are finally proved by hydrostatic testing. Records of preheat, interpass and heat treatment temperatures are retained with the weld history for the life of the unit.

Frequently Asked Questions

Q: What preheat temperature is used for T92 pipe welding?
Preheat is controlled at 100-200 °C, measured with an infrared thermometer, and the joint is soaked for at least 3 minutes before the arc is struck.

Q: What is the maximum interpass temperature for Grade 92?
250 °C, and it must never drop below the preheat temperature once welding has started.

Q: Must the root pass be purged with argon?
Yes. Internal argon at 8-10 L/min is injected through the groove gap and the foil is peeled back section by section until the root has cooled.

Q: Which filler metals match T92?
ER90S-B9 wire for gas tungsten arc welding and E9015-B9 covered electrodes for shielded metal arc welding, both classified for 9Cr-2W steel.

Q: What arc energy limit applies to the filler passes?
Arc energy is held to 22 kJ/cm maximum, with layer thickness not more than 2.5 mm and single bead width not more than 10 mm.

Q: Is post-weld heat treatment required after welding T92?
Yes. Grade 92 weldments are treated in the region of 730-770 °C before service, following the holding parameters set by the design code.