Torque and bolting

Bolting Below Freezing: How Cold Changes Torque and Preload

In the cold, the same torque value can give less real clamping force. In a 2007 study by the Korea Electric Power Research Institute, the nut factor (K) of A490 bolts rose from 0.159 at 20 °C to 0.179 at −10 °C, a 12.6% increase. A torque value calculated at 20 °C and applied at −10 °C would therefore give about 11% less clamping force. None of the sources we reviewed provides a numerical correction to apply: you need to verify, not guess.

Here is what you need to know about cold weather bolting: what a Canadian winter changes when you tighten a flange, a stud or a structural bolt, and how to keep your bolting reliable.

In short

  • Cold increases friction in the threads and under the nut. At the same torque, the bolt is less stretched.
  • None of the public references we reviewed gives a “winter percentage” to add to torque. Don’t correct by guesswork.
  • Tightening cold and then running hot (or the reverse) changes the load, especially when the bolt and flange are different metals.
  • Below a certain temperature, ordinary steel becomes brittle. “L” grade bolting (A320 L7) is made for this.
  • Hydraulic pump oil thickens, and lithium batteries must not be charged below 0 °C.

Torque is not clamping force

What keeps a flange sealed is preload: the force with which the bolt, stretched like a spring, presses the parts together. Torque is only the turning effort applied to the nut. It is an indirect way to get preload.

The link between the two is a simple formula: torque = K × diameter × preload. The nut factor K sums up all the friction. The higher it is, the more torque you need for the same preload. NASA’s Fastener Design Manual (1990) warns that the commonly used value of 0.2 “should not be used blindly” and suggests about 0.15 for steel on steel.

The surprising part: according to figures in Wikipedia’s Bolted joint article, about 50% of the torque goes to overcoming friction under the nut, 40% to thread friction, and only 10% actually stretches the bolt. Anything that changes friction changes preload. Cold is one of those things.

Does cold weather affect bolt torque?

Yes. As temperature drops, friction rises and more torque is needed to reach the same preload. The only quantified study we found, conducted in 2007 by the Korea Electric Power Research Institute on ASTM A490 structural bolts, measured a nut factor 12.6% higher at −10 °C than at 20 °C.

Kim, Nah and Lee tightened 10 sets of M20 bolts at each temperature, from −10 °C to 50 °C. Their results:

Temperature Average nut factor K
20 °C 0.159
0 °C 0.173
−10 °C 0.179

To reach the target tension of 178 kN, the required torque went from about 551 N·m at 50 °C to about 639 N·m at −10 °C. The authors conclude that when tightening by torque with the product sheet’s coefficient, the “required strength may be not ensured at a low temperature.”

Keep the study’s limits in mind: a single study, structural bolts (not flange studs), and no data below −10 °C. It shows the direction of the problem, not a rule to apply everywhere.

Should you increase bolt torque in winter?

Not without data. None of the references we reviewed (NASA, Sandia National Laboratories, the public parts of ASME PCC-1) gives a temperature correction for torque. Adding 10% “because it’s cold” can overtighten a bolt if the lubricant performs well in the cold. The right answer is to measure, or to reduce the role friction plays.

Sandia National Laboratories’ guideline (2008) recommends running any calculation with a range of nut factors rather than a single value. In practice, you have three options:

  • Ask for the lubricant’s data sheet. A well-chosen thread compound (or anti-seize) keeps friction more consistent. Check its application temperature range.
  • Warm up the work area when possible (enclosure, portable heater) to tighten in the conditions the procedure assumes.
  • Choose a method that depends less on friction, such as hydraulic tensioning or bolt elongation measurement.

Accuracy varies widely from one method to another. According to the Industrial Fasteners Institute table reproduced by NASA (1990):

Method Preload accuracy
By feel ± 35%
Torque wrench ± 25%
Turn of nut ± 15%
Load indicating washers ± 10%
Bolt elongation measurement ± 3 to 5%

Because friction changes with temperature, a torque-based method is more exposed to the cold. That is also why a properly calibrated tool matters: our article on torque tool calibration explains how often to do it. To read the pressure-to-torque conversion of a hydraulic wrench correctly, see our guide to reading a HYTORC torque chart.

Tighten cold, run hot: thermal expansion

Metal shrinks as it cools and grows as it heats. Steel moves by about 11 to 13 millionths of its length per degree. A 300 mm stud going from −30 °C to 20 °C grows freely by about 0.18 mm. That seems small, but preload comes from a stretch of exactly that order.

If the bolt and flange are the same steel at the same temperature, they move together and the load changes little. The problem comes from the difference: two different metals, or a flange heated by the process while the bolts stay cold. Sandia’s guideline (2008) puts it this way: a change in temperature can increase or decrease bolt preload. NASA (1990) adds that differential contraction can unload the joint, to the point of opening it.

In aluminum smelters, pulp and paper mills and mines, this is a typical case: a steam line tightened in the middle of winter during a shutdown, then put back in service. Plan a check pass after the equipment reaches temperature, according to the gasket type and your procedure.

What is the difference between A193 B7 and A320 L7?

The same steel, but guaranteed for the cold. ASTM A193 B7 and A320 L7 bolts use a similar chromium-molybdenum alloy and the same minimum strength (860 MPa up to 2½ in. in diameter). The difference: L7 must pass a Charpy impact test at −101 °C. B7 has no mandatory impact test.

Why it matters: below a certain temperature, ordinary steels go from ductile (they bend before breaking) to brittle (they snap without warning). The Charpy test measures that brittleness: a small notched specimen is struck with a pendulum and the absorbed energy is measured. The ASTM A320 standard covers bolting “for low-temperature service.”

The minimum temperature allowed for B7 without impact testing depends on the applicable design code and its edition. Check with your engineer before using B7 studs on outdoor equipment exposed to extreme cold.

What happens to hydraulic oil in cold weather?

It thickens, so use an oil that stays fluid at startup. In the cold, oil viscosity rises. The pump struggles to draw it in, which can cause cavitation: vapour bubbles form and damage the pump. Oils with a high viscosity index, classified HV in ISO 6743-4, are designed for outdoor use.

As John Pasko explains in Precision Lubrication (2023), oil must be light enough at very low starting temperatures to be easily drawn into the pump. Pump manufacturers set the acceptable viscosity ranges. For a HYTORC hydraulic wrench pump, follow the equipment documentation or ask our team which oil to use and how to store pumps and hoses in winter.

A pump that builds pressure slowly in cold weather is not necessarily faulty. But if the problem persists once the oil is warm, have it checked.

Can you charge lithium-ion tool batteries below freezing?

Not a standard lithium-ion battery. According to Battery University, consumer-grade lithium-ion batteries cannot be charged below 0 °C. Lithium then deposits as metal on the electrode (lithium plating), which damages the battery permanently. Ideal charging is between 10 °C and 30 °C. Some special cells tolerate charging slightly below freezing: follow the tool maker’s data sheet.

In use, most batteries drop to about 50% of their performance at −20 °C, again according to Battery University. The good news: this loss is temporary and comes back when the battery warms up. Keep batteries warm (truck, heated room, inside pocket) and let them return to room temperature before charging.

Hands lose precision too

Cold doesn’t only affect steel. Quebec’s workplace health and safety board (CNESST), in its cold stress program (2026), notes the loss of finger dexterity and sensitivity. It also flags a risk of musculoskeletal disorders when the cold forces workers to grip a tool harder.

The CNESST recommends alternating work and warm-up periods, with 10-minute breaks in a heated area, insulating metal handles and checking the wind chill index every day. A powered or hydraulic tool also reduces the effort required from the hands.

Cold weather bolting checklist

  • Does the bolting procedure specify a temperature range? If so, are you within it?
  • Is the lubricant rated for the site temperature (data sheet in hand)?
  • Are the studs a cold-service grade (L7 or another, depending on the code)?
  • Are the tools calibrated and stored warm until use?
  • Is the hydraulic pump oil suitable for cold starts?
  • Are batteries charged indoors and kept warm?
  • Is a check pass planned after the equipment reaches operating temperature?
  • Are the crew’s warm-up breaks scheduled?

Need the right tool for a winter shutdown without buying it? See our bolting tool rental options.

Sources

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