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What is Tempilstik®? — The Complete Field Guide for Welding Engineers

Published 03 March 2025 · Fast Group Engineering · 9 min read

In industrial welding, the question "is the steel surface hot enough?" must be answered with data — not guesswork. Tempilstik® is a direct-contact surface temperature indicating crayon that allows Welding Inspectors and QA/QC Engineers to confirm preheat temperature, interpass temperature, and PWHT compliance on-site — with no power supply, no calibration, and no emissivity errors.

Tempilstik® temperature indicating crayon — Made in USA, used for preheat and interpass temperature verification at industrial welding sites
Tempilstik® — surface temperature indicating crayon (Made in USA) for preheat and interpass verification at industrial welding sites.

How Tempilstik® Works — the Physical Principle

Tempilstik® is a surface temperature indicating crayon manufactured by Tempil, a brand of Illinois Tool Works (ITW), in the United States. Each crayon is formulated from a chemical compound with a precisely controlled melting point, accurate to ±1% Fahrenheit (±3% Celsius) per the manufacturer's quality standard.

The operating principle is entirely physical — no electronics, no optics:

Surface Condition What Happens Interpretation
Below rated temperature Crayon leaves a solid white chalk-like mark on the steel surface. The mark stays solid. Surface has not reached the required temperature.
At or above rated temperature Crayon melts immediately into a clear liquid on contact with the surface. Surface has reached or exceeded the threshold. Unambiguous go/no-go signal.

The melt mark leaves a clear visible trace on the surface that can be photographed as QA/QC documentation — no additional data-logging equipment required.

Key advantage over infrared pyrometers: Tempilstik® measures by direct contact — it is unaffected by the surface emissivity of the steel. On surfaces with mill scale, rust, or primer coat, infrared guns can read 5–20°C low. Tempilstik® is unaffected by any surface condition.

Temperature Range & Part Number System

Tempilstik® covers 116 temperature ratings from 100°F (38°C) to 2000°F (1093°C), each with a unique part number. The part number is printed directly on the crayon body and packaging — it is the only reference you need when ordering. Each box contains 10 crayons; individual crayons are also available.

Part Number °F °C Typical Preheat Application
#28006 150°F 66°C AWS D1.1 Group I–II thin structural steel — best-selling SKU
#28009 175°F 79°C ASME B31.3 P-No.1 pipe wall ≥ 25 mm, A106 Gr.B
#28312 230°F 110°C AWS D1.1 Group I heavy section, Group II–III thick plate
#28019 250°F 121°C AWS D1.1 Group II plate 20–38 mm, offshore structural
#28318 302°F 150°C AWS D1.1 Group III–IV, ASME P-No.4 (P11, P12)
#28327 392°F 200°C ASME P-No.5A (P22), A335 P91, Cr-Mo alloy piping
#28039 450°F 232°C Stress relief PWHT for carbon steel, ASME VIII pressure vessels
#28047 600°F 316°C PWHT for Cr-Mo steels P91/P22, power plant equipment

Browse all 116 part numbers (filterable by °C or °F) on the Tempilstik® product page. If your WPS specifies a temperature that falls between two available ratings, always select the lower of the two — this ensures you do not under-read the actual temperature relative to the minimum requirement.

5-Step Field Procedure — AWS D1.1

Using Tempilstik® is straightforward, but measurement location and timing directly affect result reliability. The following procedure conforms to AWS D1.1 Section 5 and equivalent ASME requirements.

Welding inspector using Tempilstik® to verify preheat temperature at a steel fabrication site
Verifying preheat temperature with Tempilstik® at an industrial fabrication site.
  1. Confirm the required temperature from the WPS.
    Refer to the approved Welding Procedure Specification — locate the "Preheat Temperature" or "Minimum Preheat" value. This is the minimum temperature the base metal must reach, not a target. Select the Tempilstik® part number matching that temperature exactly.
  2. Mark the surface before heating begins.
    Draw a thin line on the steel at the measurement location before applying heat. This pre-applied mark will liquefy visibly when the temperature is reached. No firm pressure is needed — Tempilstik® is softer than a pencil; normal hand pressure is sufficient.
  3. Measure at the correct position — at least 75 mm from the weld edge.
    AWS D1.1 requires temperature measurement at a minimum of 75 mm (3 inches) from each side of the weld centerline, at a minimum of 4 evenly distributed points along the weld. This position represents the temperature of the full heat-affected zone (HAZ), not the local temperature at the heat source tip.
  4. Allow adequate soak time.
    For plate thicker than 50 mm, surface temperature reaching the threshold does not guarantee the core is fully heated. Maintain the heat source for an additional 2–5 minutes after the surface reaches the target (the exact time depends on thickness) to ensure heat has penetrated the full cross-section before welding begins.
  5. Record the result in the QA/QC file.
    Photograph the melted mark at the measurement location. Record the Tempilstik® part number and time of measurement in the inspection report. A melt photograph is accepted as visual evidence by most EPC and O&G project QA systems.
Tempilstik® crayon melting on steel surface — confirmation that the required preheat temperature has been reached
When the surface reaches the rated temperature, Tempilstik® melts immediately — an unambiguous pass signal that can be photographed for the QA/QC record.

Common Mistakes to Avoid

The temperature immediately at the torch tip or induction coil is far higher than the actual HAZ temperature. Taking a reading there will give a false "pass" result while the weld zone itself is still well below the required minimum. Always measure at least 50 mm away from the heat source.

Temperature is not uniform along a long weld — especially on complex structures with bulkheads or stiffeners that act as heat sinks. A minimum of 4 evenly distributed measurement points is required per AWS D1.1 for each weld pass.

If the WPS requires 150°C (302°F) but the inspector uses a 200°C (392°F) crayon, the surface may have already met the 150°C requirement — but the higher-rated crayon will not melt, giving a false "fail" result. Always verify the part number against the WPS minimum preheat before each welding shift.

After the first weld pass, interpass temperature must also be controlled — it must not exceed the WPS maximum. Use a higher-rated Tempilstik® (e.g. 300°C / 572°F) to verify the surface has not exceeded the maximum interpass limit before depositing the next pass.

Crayons left in a vehicle parked in direct sunlight or near a heat source can deform at the tip, which may affect the melting response. Store in a cool, dry location away from temperatures above 40°C (104°F).
⚠️ Maximum interpass temperature — P91 and high-alloy steels:
For P91 steel (A335 Gr.P91) and other high-alloy materials, AWS and ASME specify a maximum interpass temperature — typically 300°C (572°F / 482°C per #28053). Exceeding this limit alters the weld microstructure. Use Tempilstik® #28053 (900°F / 482°C) to confirm the surface has not exceeded this threshold before depositing each subsequent pass.

Preheat vs Interpass vs PWHT — Key Differences

The same Tempilstik® crayon serves three distinct temperature control purposes in the welding sequence:

Application When to Measure Purpose Typical Temperature Range
Preheat Before striking the arc Confirm base metal ≥ WPS minimum — reduces cold cracking risk in HAZ 50–250°C (carbon steel)
150–350°C (alloy steel)
Interpass Between weld passes Confirm surface does not exceed WPS maximum — prevents toughness loss and adverse microstructure changes ≤ 200–300°C depending on material
PWHT verification During heat treatment cycle Confirm surface has reached the required soak temperature 600–760°C (stress relief)
700–760°C (P91)

For PWHT at elevated temperatures (above 600°C / 1112°F), Tempilstik® remains fully functional — the range extends to 2000°F (1093°C). However, for industrial furnace PWHT where temperature must be continuously recorded, best practice is to use a thermocouple system as the primary record and Tempilstik® for independent spot-check verification.

Tempilaq Advanced liquid and Tempilstik crayons used together to control both minimum preheat and maximum interpass temperature simultaneously
Using Tempilaq® Advanced and Tempilstik® together allows simultaneous control of minimum preheat and maximum interpass temperature in one operation.

Accepted Standards

For QA/QC Engineers documenting measurement methods in inspection records, Tempilstik® is recognized as a valid direct-contact temperature measurement method by the following standards:

Temperature indicating crayons listed in these standards do not require periodic recalibration schedules because the measurement mechanism is chemical — not electronic — and therefore has no drift requiring correction.

Selecting the Right Part Number from Your WPS

In practice, WPS documents state preheat temperature in Celsius. The fastest method to select the correct part number:

  1. Read the minimum preheat temperature from the WPS (°C)
  2. Convert to °F: T°F = T°C × 9/5 + 32
  3. Find the Tempilstik® part number whose rating is closest to — but not lower than — the required temperature
  4. If two adjacent ratings straddle the requirement, select the lower one to detect the precise threshold
Practical example: WPS requires minimum preheat 100°C → convert: 100 × 9/5 + 32 = 212°F → select Tempilstik® #28016 (225°F / 107°C). When the crayon melts, the surface has precisely reached 212°F.

→ Browse the complete 116-SKU list at the Tempilstik® product page — filterable by °C or °F.
Available as a box of 10 crayons or individually. Stock held in Ho Chi Minh City and Vung Tau.

Related products for complete temperature control:

  • Tempilaq® Advanced — liquid temperature indicator, ideal for marking large areas or monitoring maximum interpass temperature
  • Tempilabel® — self-adhesive temperature indicator labels for equipment monitoring and continuous surface surveillance
  • Thermomelt® — fusible pellets for furnace and oven temperature verification

Frequently Asked Questions

No. Tempilstik® works on a chemical principle — the melting point is set at the factory and does not drift over time with correct storage. There are no electronic components and no mechanical parts that require inspection. This is a significant advantage over electronic instruments when completing QA audit documentation.

Yes. The crayon functions correctly in wet or humid conditions because measurement is by direct contact — not optical or electronic. Note that wet steel surfaces lose heat faster once the heat source is removed, so readings should be taken immediately. Strong wind also accelerates heat loss — factor this in when maintaining preheat outdoors.

Typically 20–40 marks per crayon, depending on application pressure and mark length. The crayon advances from its metal holder as used — similar to a pencil — and the remaining compound is protected from breakage by the holder.

Current-generation Tempilstik® (manufactured from 2012 onwards) is lead-free and sulfur-free, as stated on the box label. Normal skin contact under standard conditions is not considered hazardous. Avoid contact with face and eyes. Full composition details are in the Safety Data Sheet (SDS) supplied with genuine product.

Yes — Tempilstik® works on austenitic stainless steel (304, 316) and duplex stainless. However, note that sulfur in older crayon formulations could cause surface contamination on stainless at elevated temperatures. The current sulfur-free formulation resolves this issue — but review the SDS before using on pressure-critical stainless steel systems.

Need to order Tempilstik® for your project?

Fast Group Engineering is the authorized Tempil® distributor in Vietnam — direct import from the USA, with C/O (Certificate of Origin), C/Q (Certificate of Quality), and VAT invoice. Available as a box of 10 or individual crayons.

Request a Quote Browse All Part Numbers

📞 +84 938 888 958  |  ✉ sales@tempil.vn

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P91 Steel Welding — Preheat 200°C & PWHT 730–760°C

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P-Number classification and preheat requirements for process piping systems.