The hot forming, either by forging, pressing or rolling, must take into account: the behavior of each steel during warm, work itself, and subsequent cooling.
This requires some prior experience as steels of different compositions expand and contract differently in the heating and / or cooling from the forging temperature. The forged high alloy steels is very delicate and we must abide by specifications the manufacturer.
structurally in solid solution, steels have the highest malleability, that is composed of steel beads austerita. This grain is a solid solution of iron carbide in iron range (magnetic iron). The appearance of austenite in the heating of ordinary steel, carbon occurs above 720 ° c (Ac1 or critical point below) and ends to become completely between this and the 910 º C, depending on this percentage of carbon in composition. In this final transformation point is called critical point above Ac3.
Pure iron (0% carbon) is the point Ac1 = 720 ° c, and Ac3 = 900 ° c. While in the steel of eutectic composition (0.9% carbon) in austerita transformation occurs at a single temperature of 720 º C approx.
These temperature ranges are important, both for casting and for any heat treatment applied to steel.
temperature material, the beginning of the slab should be about
A too rapid and uneven heating, or degree of deformation with a too steep and uneven cooling of the floor too quickly can cause stress cracks and deformations.
Up to approx. 800 º c, the heating must be slow and uniform. You can then rise more rapidly to the final temperature. Forging temperature is reached, proceed rapidly to deformation; exceed or maintain the same temperature for too long, it causes a coarse-grained. The steel is fragile, it takes less quenching, can be superficially decarburized and general properties are diminished physical.
After forging parts must be cooled slowly and evenly, for example. dry ash.
Annealing
After many steels, particularly steel, are too harsh for a further mechanization and must be annealed to achieve a certain degree of softening.
Depending on the aim sought by the annealing for a carbon steel of 0.60% of this, the temperature can range between 680 º c to 800 º c.
If the steel is maintained for a long time at higher temperatures the upper critical (Ac3), the austenite crystals tend to grow and grow. This temperature increases with higher this, and the longer the duration of heating, and as the grain size after the treatment is very dependent on the size had austerita crystals, we get a rude and a steel structure of low features. When this happens, to tune, just heated to a temperature more just above the upper critical (Ac3) and then cooling more or less quickly into the air as the composition
Temple
The temple is an operation that is to cool the steel by heating something beyond its upper critical point (Ac3) in a medium that will remove the heat, quickly as possible. This means that the steel must be in state austenitic (nonmagnetic) and this product have been heated by about 720 ° c, depending the final temperature of the chemical composition of steel.
To produce austerita-martensite transformation, the cooling rate should be equal to or above the "critical speed" cooling for the steel in question. As such, this varies with the chemical composition of each steel.
quenching effects on carbon steels
a) increases the hardness from 1.5 to three times.
b) Increases resistance almost 50%.
c) Decreases elongation, shrinkage, workability and toughness.
d) Tune the grain.
e) can lead to distortions in the piece.
f) can cause cracking and breaking.
g) slightly increases the volume of the piece.
h) increases the electrical resistance.
factors that determine a good temper
a) The cold-worked pieces (temp.menor to 500 degrees c) forged or were warm, have a very homogeneous structure, so whereas before warm them practicable
annealingb) Percentage of carbon: the quantity of this metalloid has great influence on the hardness of hardened steel that acquires, which contains 0.1% almost no hardening, while those containing 1.2% carbon, high strength gain .
A steel that is already hard, raise their hardness with low intensity of hardening, while maintaining their tenacity to get the same hard drive with a steel less open to employ more aggressive cooling bath, which ara more brittle steel.
c) Heating: the heating rate must be paramount, taking care that the piece reaches its thermal equilibrium, without which the carbon does not enter into solid soluduion the austerita causing a surface hardening.
d) Cooling rate: the hardness acquired by the tempered steel, is greater the faster it cools. This speed depends on the nature of steel, part dimensions, nature and temperature of cooling bath, how he keeps in the bathroom part for tuning, and the surface state of the pieces.
Tempering
Tempering is the heat treatment is carried out after quenching, and involves heating the hardened steel at temperatures below the critical point lower (Ac1-720 º c) and oil quenching or air. It is advisable to do it immediately after quenching to avoid the danger of breakage due to internal tensions.
The purpose of this treatment are:
a) Delete molecular tensions arising in the temple.
b) increase the toughness and tempering losses.
c) To improve the mechanical properties of materials and mix refined structures.
annealing treatment increases the toughness, because the structure becomes harder hardening (martensite) in a less fragile, but the hardness decreases.
factors that determine the characteristics of tempering, the temperature and exposure time to this.
temperature allows the material to reach the degree of toughness related to the work to be done, the exposure time makes the transformation of the structure, reach the heart of the piece.
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