Archive for July, 2011

Weldability of Structural Steels

 

If weld preparation is good and operator induced defects (e.g. lack of penetration or fusion) are avoided, all the common structural steels can be successfully welded. However, a number of these steels may require special treatments to achieve a satisfactory joint. These treatments are not convenient in all cases.

Welding

The difficulty in producing satisfactory welded joints in some steels arises from the extremes of heating, cooling and straining associated with the welding process combined with microstructural changes and environmental interactions that occur during welding. It is not possible for some structural steels to tolerate these effects without joint cracking occurring. The various types of cracking which can occur and the remedial measures which can be taken are discussed below.

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Inclusions Phenomena at Steelmaking

Sulphur, Phosphorus and Other Impurities

One tonne of steel, a cube with sides of about 0,5m, contains between 1012 and 1015 inclusions which can occupy up to about 1% of the volume. The total content is largely determined by the origins of the ores, coke and other materials used to extract the metal in the first place, and by the details of steelmaking practice.

The principal impurities which worry steelmakers are phosphorus and sulphur. If not at very low concentrations, these impurities form particles of phosphide and sulphide which are harmful to the toughness of the steel. Typically, less than 0,05% of each of these elements is demanded. Low phosphorus contents are relatively easily attained during the refining of the pig iron into steel, but sulphur is more difficult to remove. It is controlled by careful choice of raw materials and, in modern steelmaking, by extra processing steps to remove it.

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Rapidly Cooled Steels

Formation of martensite and bainite

Normalising causes steels to undercool below the requirements of the phase diagram before the austenite transforms into fine ferrite and pearlite. Still further increases in cooling rate give further undercooling and still finer microstructures. Very rapid cooling by quenching into cold water, causes the formation of ferrite and pearlite to be suspended.

Longitudinal section of hot rolled structural steel showing dark bands of pearlite in a ferrite matrix.(x 200)

The internal diffusion-controlled rearrangement of atoms needed to form those products cannot occur sufficiently rapidly. Instead, new products are formed by microstructural shear transformations at lower temperatures. Very fast cooling gives martensite: its microstructure is shown above. When martensite forms, there is no time for the formation of cementite and the austenite transforms to a highly distorted form of ferrite which is super saturated with dissolved carbon.

The combination of the lattice distortion and the severe work hardening resulting from the shear deformation processes necessary to achieve the transformation cause martensite to be extremely strong but very brittle. Less rapid cooling can give a product called bainite. This is similar to tempered martensite where much of the carbon has come out of solution and formed fine needles of cementite which reinforce the ferrite.

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Cooling Rate

Cooling Rate During Austenite to Ferrite Transformation and Grain Size

During cooling of austenite, the new bcc ferrite crystals start to grow from many points. The number of starting points determines the number of ferrite grains and consequently the grain size. This grain size is important because the engineering properties are dependent on it. Small grains are favourable.

Slide 7 : Microstructure of pearlite. (x 1000)

By adding elements like aluminium and niobium, the number of starting points can be increased. Another important factor is the cooling rate. When cooling is slow, the new ferrite grains develop from only a few most favourable sites. At high cooling rates, the number of starting points will be much higher and the grain size smaller. Slides 7 - 9 shows steels with various grain sizes, produced at different finish rolling temperatures.

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    • Weldability of Structural Steels July 4, 2011
      If weld preparation is good and operator induced defects (e.g. lack of penetration or fusion) are avoided, all the common structural steels can be successfully welded. […]
      metalkunde
    • Inclusions Phenomena at Steelmaking July 4, 2011
      One tonne of steel, a cube with sides of about 0,5m, contains between 1012 and 1015 inclusions which can occupy up to about 1% of the volume. The total content is largely determined by the origins of the ores, coke and other materials used to extract the metal in the first place, and by the details of steelmaking practice. […]
      metalkunde
    • Rapidly Cooled Steels July 2, 2011
      Formation of martensite and bainite Normalising causes steels to undercool below the requirements of the phase diagram before the austeniteRead the Rest... […]
      metalkunde
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