Equipment for local and volumetric PWHT (heat treatment of welded joints)

Methods for temperature determination without thermocouples

25.09.2020 15:27:00

In non-professional practice, several methods can be used to determine temperature of heated part without measuring instruments.

The first method is "by glow color"

Steel emits light rays of different colors depending on the heating temperature when it is heated up to 530°С.

Heat steel color Heating temperature, °С
Dark brown (visible in the dark) 530-580
Brown red 580-650
Dark red 650-730
Dark red cherry 730-770
Cherry red 770-800
Light redcherry 800-830
Light red 830-900
Orange 900-1050
Dark yellow 1050-1150
Light yellow 1150-1250
Dazzling white 1250-1350

Colors of hot steel and their temperatures are given in table. Colors are corresponded to daylight.

It is important to remember, that ambient light conditions (bright daylight, low artificial or natural light) are influence to accurate definition of steel temperature. They distort real color. Besides that, this mathod can't be accurate because of individual features of observer's eyes. For expirienced one, mistake is less then 30 dergees.

The second method "by tempering colors"

Tempering colors appear on a surface cleaned with emery and heated up to 200-300 degrees (table "Temper colors"). It happens due to the formation of oxide films of various densities; every film reflects rays of certain color.

Temper color Heating temperature, °С
Light yellow 220
Straw yellow 240

Brown yellow 255
Red brown 265
Purple red 275
Purple 285
Cornflower blue 295
Light blue 315
Gray 330

Matches method

Matches are the easiest and affordable way to get fire. You can use matches to define temperatures of preheating before and during welding.

Matches are often made of aspen, linden, poplar or american pine. We should mark, that most of russian matches are made of aspen. Its head is made of berthollet salt and potassium chromium peak, which give oxygen in high temperature. Pyrolusite catalyst is added to limit temperature. Also, watches are consist of sulfur, glue and phosphorus sulfide, that make a watch to burn. Ground glass, zinc white and red lead are added to control burning speed.

Temperature control during heat treatment

Temperature is required to ignite the sulfur head of a match exceeds more than 180 degrees Celsius. If you touch head of a match to heated metal with such temperature, it will light up.

Saliva method

Students learn that water boils at a temperature of 100 degrees Celsius in the earliest physics lessons at school. Therefore, by dropping water on the heated metal (in any cases, by spitting on the pipe), you can determine is pipe heated to a temperature above 100 degrees or not.

A drop of water during heat treatment

Despite the abundance of non-instrumental control methods, their application requires extensive experience. But in any case, all these methods have a fairly high error. Therefore, whenever possible, it is recommended to use high-precision certifiedtemperature recorders, thermocouples orpyrometers.

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