Конструкциялық материалдар және термоөңдеу Конструкционные материалы и термообработка Constructional materials and heat treatment Учебное пособие для специальности: 5В071200– «Машиностроение»

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Depending on temperature of heating distinguish: 
- full hardening at which heating is carried out in single-phase austenitic area 
(on 30-50z higher than the A3 line). At fast cooling there is a full transformation 
of austenite into marten site. 
- incomplete hardening at which heating is carried out in two-phase area 
(between the A3 and A1 lines). 
In practice full hardening apply for proeutectoid steel, incomplete for 
hypereutectoid steel. 
Heating temperature under training alloyed steel usually above, than for the 
carbonaceous. Diffusive processes in alloyed steel proceed more slowly 
therefore they require longer endurance. 
For achievement of the maximum hardness when training seek to receive 
marten site structure. The minimum speed of cooling necessary for overcooling 
of austenite before marten site transformation, is called as the critical speed of 
hardening. Speed of cooling is defined by a type of the cooling environment. 
Usually for hardening is used boiling liquids: 
- water. 
- water solutions of salts and alkalis. 
- oils. 
The choice of the concrete hardening environment is defined by a type of a 
product. For example, water with a temperature 18-25C use generally when 
hardening details of a simple form and the small sizes executed from 
carbonaceous steel. Details of more difficult form from carbonaceous and 
alloyed steel temper in oils. For training alloyed steel often use water solutions. 
Important characteristics steel, necessary for purpose of technological modes 
of training, harden ability and a hardness penetration. 
Harden ability characterizes ability of steel to hardness increase when 
hardening depends mainly on the content of carbon in steel. Harden ability 
estimate on the hardness of a blanket of a steel sample after training. 
The hardness penetration characterizes ability began to be tempered on the 
demanded depth. The hardness penetration is estimated on distance from a 
product surface to a layer which contains not less than 50% of marten site. 
Ways of hardening steel: 
- hardening in one cooler at which the heated detail plunges into cooling liquid 
and remains there before full cooling.
The simplest way. Shortcoming - emergence of considerable internal tension. 
- hardening in two environments at which a detail to 300-400zs cool in water, 
and then transfer to oil. Apply to reduction of internal tension at heat treatment 
of products from tool high carbon steels. Shortcoming - difficulty of regulation 
of endurance of details in the first environment. 
- step hardening at which the detail is quickly cooled with immersion in a salt 
bath with a temperature which is a little exceeding temperature of marten site 
transformation, is maintained before achievement of identical temperature on all 

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section, and then cooled on air. Slow cooling on air reduces the internal tension 
and possibility of a buckling. Shortcoming - restriction of the size of details. 
- isothermal hardening at which the detail is maintained in a salt bath before the 
end of isothermal transformation of austenite. 
- hardening with self-drawback at which in the hardening environment cool only 

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