By changing the temperature of steel by color is understood the range of shades that can be formed on metal surfaces during the appearance of oxide film. These oxide films are formed from the metal itself when heated. A key condition for the appearance of such a film is considered to be the absence of the impact of the aquatic environment on the surface.
How do colors appear when a metal is heated?
Color changes in the temperature of steel are a special range of colors that can be formed on the exterior coatings of various iron alloys due to the formation of oxide films. The latter appear during the warming up of metal surfaces to the specified temperatures (during the procedure water is not involved). It is necessary to focus on the fact that the colors of the temperature change of steel are considered a flaw of the resulting weld.
It is not possible to calculate the temperature of the metal alloy with the help of the tones of the steel, as many variables influence this temperature (quite precisely).
To these should include:
- presence of different impurities and additional components in welded metal alloys
- warm-up period (the time interval during which the metal products are heated to the outside temperature in the absence of heat recovery)
- Heating rate (change of temperature per unit during warm-up)
- degree of illumination of the area where the workpieces were hardened or welded
In the industrial sphere, temperature control is carried out by means of specialized devices called pyrometers. Devices are supplemented with special sensors that determine the degree of heating of the part with the help of laser
In the absence of instrumental control, structural defects such as fragility and overheating arising from electric arc and contact welding, many experts also determine at the external inspection of the product, studying the traces of the temperature of steel. Thus, silver shades are considered to be an indicator of correct technology and good protection of stainless steel weld metal and carbon steel.
Rainbow shades of the blue-violet spectrum indicate a lack of technology, and they arise from the intense heat distribution from the central part of the heating spot and speak of a decrease in plasticity and impact viscosity of steel.
The metal adjacent to the electrically active spot is heated mainly by radiant heat exchange with an arc column and convection heat exchange with hot gases. The distribution of the specific heat flow over the heating spot radius can be roughly described by the Gauss probability distribution law:
where
q2max is the largest specific heat flow in the center of the heating spot;
k – is the ratio of concentration of the specific flow of the arc, cm-2;
r – distance from the source axis, cm.
With an increase in current at DC voltage, q2max increases, and with an increase in voltage (arc elongation) and a constant current, the specific heat flow decreases, and its distribution becomes less concentrated.
The use of fluxes and gas protection mixtures makes it possible to concentrate the heat flow in a smaller area and thus avoid the appearance of brittleness – a reduction in the plasticity of steel with a simultaneous increase in strength and the manifestation of blue colors of metal.
Features that affect the efficiency of oxide film formation
We emphasize that oxide films on metal blanks are formed at different speeds.
It depends on:
- Type of product (more rapid handling if tempered elements are used)
- Roughness. Products with significant irregularities get too thick film. Thanks to which an attractive transfusion of tones can be overlooked. In the case of polished products, their surfaces quickly form the thinnest equal oxide layer
- presence of dirt, other unnecessary components. When warming up all external elements (dirt, rust, oil layer) are charred, which makes the procedure of forming a uniform film more difficult
- process characteristics of heating. Appliances used for heating workpieces also affect this process. To do this, it is better to use devices that allow you to monitor and maintain the required temperature at a stable level
Note that the color change temperature of the metal may vary for each individual type of alloy. Therefore, there are a lot of lists and tables where temperature and shade ratios are reflected.
How to Artificially Create Metal Color Change?How to Artificially Create Metal Color Change?
You can get different shades of color change on steel, titanium, copper and bronze by thermal and chemical way. The first influences the structure and physical-mechanical properties of the metal being treated, the second is essentially chemical coloring, in the process of which the substance of the product being painted can take part
Thermal method
To obtain the desired shade, the articles are placed in a muffle furnace or heated with a gas burner. As a result of intense heat action on the metal surface will begin to change colors quite quickly and here the main thing – to stop heating at the moment when the product has the desired color.
Chemical method
Using various chemicals and compounds, it is possible to achieve the formation of films that cause the appearance of interference colors and at the same time to get very bright and diverse shades not only on titanium and steel, but also on silver, nickel, copper, and bronze and brass. Another thing is that the first colors on copper and bronze surfaces are slightly changed due to the color of the metal itself and slightly different from the colors on titanium and iron alloys.