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Characteristics of vacuum heat treatment of quenched and tempered steel
Quenched and tempered steel generally refers to medium carbon steel or medium carbon alloy steel used after quenching and tempering (vacuum hardening plus vacuum tempering).
Performance requirements of quenched and tempered steel
The working loads of many important machine parts, such as machine tool spindles, automobile and tractor rear axle half shafts, diesel engine crankshafts, connecting rods, etc., often change, and are sometimes subject to impact, and there is also intense friction in the journals or splines. The failure forms of such parts are mainly fatigue fracture, excessive deformation, and local wear. Therefore, the steel used should have high strength (yield strength and fatigue strength); good plasticity and toughness; certain surface and local resistance, etc., requiring both strength and plasticity and toughness, that is, it should have good comprehensive mechanical properties.
Vacuum heat treatment characteristics of quenched and tempered steel
(1) Pre-heat treatment
Quenched and tempered steel contains different types and quantities of alloying elements, and its structure after hot processing may be ferrite and fine pearlite, or martensite may appear. The former can be heated to above Ac3 for complete annealing (if the carbon content and alloying element content of the steel are low, normalizing can be used instead) to refine the grains, reduce the banding of the structure, and improve the cutting performance of the steel. For the latter, after normalizing, it should be tempered at a high temperature below Ac1 to reduce the hardness and facilitate cutting.
(2) Final heat treatment
The first process of the final heat treatment of quenched and tempered steel is vacuum hardening. Because quenched and tempered steel is a sub-eutectic steel, its vacuum hardening heating temperature is above Ac3, and the specific temperature is determined by the composition of the steel. The vacuum hardening medium can be selected according to the permeability of the steel and the size and shape of the parts. In fact, general alloy steel workpieces are quenched in oil.
Vacuum tempering is an important process to finalize quenched and tempered steel parts. The selection of vacuum tempering temperature is determined by the technical requirements of the parts and the chemical composition of the steel.
Usually, there are two options for the vacuum tempering temperature of quenched and tempered steel.
① High temperature vacuum tempering
Generally, a vacuum tempering temperature of 500~600℃ is used to obtain tempered martensite structure of steel, and obtain higher strength and higher plasticity and toughness. It should be noted that for steel with the tendency of the second type of temper brittleness, it should be quickly cooled in water (oil) after high temperature tempering to prevent the development of temper brittleness.
② Low temperature vacuum tempering
When the parts require very high strength (σb=1600~1800MPa), only by appropriately sacrificing plasticity and toughness and vacuum tempering at 200~250℃ can medium carbon tempered martensite structure be obtained. For example, rock drill piston, ball pin, etc.
Parts usually made of quenched and tempered steel, in addition to requiring higher strength, toughness and plasticity, often also require good wear resistance in certain parts (such as the journal or spline part of shaft parts). For this reason, after quenching and tempering, high-frequency induction surface quenching is generally performed on local parts.
For parts that require good wear resistance, tempered steel containing Cr, Mo, and Al is usually selected. After tempering, if nitriding is performed, nitrides of Cr, Mo, and Al can be formed on the surface of the workpiece, which significantly improves the hardness and wear resistance. Therefore, this type of steel is also called nitrided steel. The nitriding process is generally carried out below 600°C. The purpose of nitriding of structural steel is to improve its hardness, wear resistance, thermal stability and corrosion resistance. Before nitriding, the parts should be vacuum hardened + vacuum tempered.
The processing route of connecting rod bolts is: forging-annealing (or normalizing)-machining (rough machining)-tempering-machining (finishing)-assembly.
Annealing (or normalizing): As a pre-heat treatment, its important purpose is to improve the forging structure, refine the grains, and improve the cutting performance.
Tempering-vacuum hardening: heating temperature (840±10)℃, oil cooling, obtain martensitic structure.
Vacuum tempering: heating temperature (520±10)℃, water cooling (to prevent the second type of temper brittleness).
After quenching and tempering, the structure is tempered troostite, and blocky ferrite is not allowed to appear, otherwise it will reduce strength and toughness. Its hardness is about 263~322HBS.
SIMUWU Vacuum Tempering Furnace
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