Metallography and Heat Treatment of Metal

The crystals shape (Part 2).

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With the increase in the growth rate of crystals, the dimensions of the cells decrease. With intensive heat removal through the solidified metal, when the growth rate of the crystals is high, the cellular structure does not arise. Thus, at the low cooling rates, the surface of the crystallization front is almost smooth. At medium – the cellular structure appears, the elements of which are crushed with the acceleration of cooling. At the high cooling rate, the branches appear in place of the cells, indicating the initiation of dendritic crystallization.

In the presence of a super cooled melt zone in front of the crystallization front, in Continue reading →

To become an Inspection Engineer

Tests to determine the chemical properties.

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Earlier we considered those types of tests that allow us to know the mechanical properties of metals. The metals also have important chemical properties. In fact, it is the chemical composition of the metal and its heat treatment that largely determines its mechanical properties. The part is necessary to determine the chemical composition of the metal. To do this, there are three common methods: spectral analysis, combustion analysis and analysis of the effects of liquid reagents.

The welding inspector rarely has to practice chemical analysis. However, the inspector may need assistance in the sampling and sampling process for analysis or Continue reading →

To become an Inspection Engineer

The power supply classification features. The potential distribution in the arc. The welding arc properties (Part 2).

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The studies have shown that the extent of the cathode and anode zones of the arc is very small: :  lк = 1·10-5 см;  lа  = 1·(10-3…10-4) cm. Then the length of the column of the arc: lс ≈ lд (equal to the length of the arc), and

Uд = Ua + Uк + Uс ,

Lд = lа + lк + lс .

The power released in the 3 zones of the arc Pд = Icв·Uа  + Icв·Uк+Icв·Uс, is completely determined by the values Uа,Uк,Uс, Uа = 2..5 V; Uк = 4..20 V. For steel electrodes Continue reading →

Metallography and Heat Treatment of Metal

The metal structure basis (Part 4).

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The metal melting in a small area, leads to the appearance of shrinkage stress. Even if the external fixation of the sample was ensured during the heating and cooling cycle, the tension caused by uneven heating and cooling persists in the cooling part. This voltage is called the residual voltage. Due to the residual stress, the sample remains curved. However, the sample will not continue to bend further, since it has already cooled to room temperature and has acquired sufficient strength to resist residual stress forces. The residual stress remains in the sample if Continue reading →

Metallography and Heat Treatment of Metal

The crystallization process mechanisms.

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The crystallization takes place in two stages: in the beginning, nucleation center arise, and then nucleation center grow. There are two mechanisms for the nucleation of the nuclei: crystallization centers:

  1. Homogeneous (spontaneous, spontaneous without extraneous particles);
  2. Heterogeneous (not spontaneous, with the participation of foreign impurities at the ready interfaces.)

The growth of crystallization centers does not depend on the method of nucleation of crystallization centers and obeys general laws.

  The homogeneous crystallization theory.Continue reading →