When preheating, it is highly recommended that preheat temperatures be measured with a calibrated, digital, electronic pyrometer with a surface contact probe.

The minimum acceptable requirement is the use of two “Tempstiks”; one for each of the minimum and maximum temperatures of the range specified in the preheat temperature.

The unit operator must ensure that the desired, preheat temperature range is within the required range.

Excessive preheat can sometimes distort the weld bead profile while insufficient preheat can produce inadequate fusion and other negative results of the finished weld.

Note: 1. Laser pyrometers may be utilized to measure preheat and inter-pass temperatures. They too must be calibrated and kept clean for proper use. They will not read correctly on a “shiny” or “deflection” metallic surface unless the lens in the unit is properly filtered. Most common hand-held units, off the shelf, do not have filters. In addition, proper use of the laser as instructed by the manufacturer is required to assure accuracy of readings.

Below is an example of reading at near HAZ (shiny surface) and on base metal.

Standard reference

Recommended temperature refer to a minimum requirement of base metal and welding electrodes where higher range is preferable application.

The preheat temperature of a fillet weld is higher than in a butt weld because of the combined thickness and therefore the heat sink.

Additional determine the minimum preheat temperature by carbon equivalent values (CE), heat input, and combined thickness as given in EN 1101-2 Annex C.

ASME B31.3 2018 Table 330.1.1

P-No. Limits Thickness Min. Temperature
1 %C > 0.3 <= 25 mm 10 Deg. C
1 %C <= 0.3 > 25 mm 10 Deg. C
1 %C > 0.3 > 25 mm 95 Deg. C

Other material see ASME B31.3 2018 Table 330.1.1

AWS D1.1 2020 Table 5.8

Steel specification Welding process Thickness (mm) Required Min. Temperature
Group I, II SMAW, with low hydrogen electrodes 3 to 20 10 Deg. C
Group I, II SMAW, with low hydrogen electrodes 20 to 38 65 Deg. C
Group I, II SMAW, with low hydrogen electrodes 38 to 65 110 Deg. C
Group I, II SMAW, with low hydrogen electrodes Over 65 150 Deg. C

Other material see AWS D1.1 2020 Table 5.8

Welding rods, electrodes and filler metal (low hydrogen)

AWS Specification Refer para./table
AWS A5.5 Table 7
AWS A5.23 Table 7
AWS A5.28 Table 7
AWS A5.29 Table 6

Preheating temperature

The minimum temperature in the weld joint preparation immediately prior to the welding; or in the case of multiple pass welds, the minimum temperature in the section of the previously deposited weld metal, immediately prior to welding.

Inter-pass temperature

The maximum temperature recorded of the previously pass/layer before apply next run.

Preheat maintenance temperature

Practice of maintaining the minimum specified preheat temperature, or some specified higher temperature for some required time interval after welding or thermal spraying is finished or until post weld heat treatment is initiated.

Measurement

The temperature measurement shall normally be made on the surface of the workpiece facing the welder, at a distance of A = 4 × t, but not more than 50 mm, from the longitudinal edge of the groove. This shall apply for workpieces thickness t not exceeding 50 mm in the weld.

The temperature shall be measured on the face opposite (other site) to that being heated.

Inter-pass temperature shall be measured on the weld metal or the immediately adjacent parent metal.

Time of measurement

Interpass temperature shall be measured in the weld area immediately before welding the next layer.

Preheat maintenance temperature shall be monitored during the period of welding interruption.

Test equipment

Equipment used for temperature measurement should be specified in the welding procedure specifications.

Temperature sensitive materials (e.g. crayons or paints) (TS): (Pocket Fold up Thermometer – Extech® 39272)

Contact thermometer (CT): (ProHeat 35 – Thermocouple)

Thermocouple (TE): (Fluke 568 – contact/non-contact thermometer)

Records

The following minimum information in accordance with the specification in welding procedure specification:

  • Measured preheating temperature, in °C;
  • Measured interpass temperature, in °C;
  • Measured preheat maintenance temperature, in °C;

Note: Soak Heat is achieved by applying a constant heat source to the weldment, uniformly apply heat to the groove bevel area plus 75mm beyond throughout the thickness of the part rather than only accomplishing surface heating. It is determined by removing out the heating device and measuring the temperature. After taking a temperature reading, immediately cover the weldment with a thermal blanket or canister and let it sit for 3 minutes. Remove the blanket or canister and measure the temperature again. If the drop is greater than 50°F (10°C), Soak Heat has not been achieved.

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