T8 steel heat treatment problem, how to quench and temper T8 steel

T8 belongs to carbon tool steel, hardened steel for plastic molds. After quenching and tempering, it has high hardness and wear resistance, but low hot hardness, poor hardenability, easy deformation, low plasticity and strength.

Standard: GB T 1298-1986

Features and scope of application:

It is used as a variety of tools that require high hardness and wear resistance, such as molds and punches with simple shapes, tools for cutting metal, drilling tools, milling cutters for woodworking, countersunk drills, axes, chisels, longitudinal hand saws, and secondary tools such as fitter assembly tools and rivet dies. It is not easy to make tools that can withstand large impacts.

T8 Carbon Tool Steel Chemical Composition:

Carbon C: Phosphorus P:

Chromium Cr: Permissible Residual Content When Fabricating Lead Bath Quenched Steel Wire) Nickel Ni: Permissible Residual Content When Fabricating Lead Bath Quenched Steel Wire) Copper Cu:

Allowable residual content When manufacturing lead bath quenched steel wire) Note: Allowable residual content cr+Ni+cu When manufacturing lead bath quenched steel wire) Mechanical properties: hardness:

Annealed 187HB, indentation diameter ; Quenched to 62hrc.

Heat treatment specifications and metallographic structure:

Heat treatment specification: specimen quenching 780 800, water cooling.

Delivery Condition: The steel is delivered in annealed condition. By agreement between the two parties, it can also be delivered without annealing.

T8 Gang 770 Keep warm for 20 minutes Air cooled Oil cooled Water cooled to room temperature What is the theoretical HRC respectively?

Air cooling: HBS 187 or so.

Oil cooling: about 62HRC.

Water cooling: about 62HRC.

It also depends on the weather temperature, water temperature, oil temperature and other specific conditions.

Normal air cooling is about 230hbs, air cooling or wind mist cooling is about 260hbs, oil cooling is 55-60hrc, and water cooling is 58-62hrc

Common heat treatment processes for T8 steel:

Spheroidization annealing after forging: heating temperature 750 770, isothermal temperature 600 650, furnace cooling to below 500 air cooling.

Normalizing: heating temperature 800 820, air cooling.

Stress relief annealing: heating temperature 650 700, furnace cooling or air cooling.

T8 or T8A steel quenching process:

1. Water quenching oil cooling, 740 750 first quench into 20 40 water, cool to 200 250, transfer to 20 40 oil and cool to room temperature hardness (HRC) 62 65. or 740 750 first quench 20 40 of 5% salt water, cool to 200 250, transfer to 20 40 oil and cool to room temperature hardness (HRC) 62 65. It can also be 740 750 quenched into 5% 10 alkaline water of 20 40 first, cooled to 200 250, transferred to 20 40 oil and cooled to room temperature hardness (HRC) 62 65

2. Oil cooling, 760 770 quenched into 20 40 oil cooled to room temperature hardness (HRC) 60 63.

3. Graded quenching (diameter size or effective thickness is not more than 12mm mold parts), 760 770 quenched into a molten nitrate bath of 170 200, isothermal 3 5min, out of the bath air cooling hardness (HRC) 60 63.

Tempering: heating temperature 160 180, holding time, hardness after tempering 58 62hrc.

Quenching and tempering treatment: quenching and heating 800, heat preservation, water cooling; Temper and heat 700, keep warm for 2h, the furnace is cooled to below 550, and the furnace is air-cooled.

Solid chromium: the composition (mass fraction) of the penetrating agent is 50% chromium powder + 44% aluminum oxide powder + 6% ammonia chloride. Chromium heating 1050, heat preservation for 8h, air cooling.

Follow-up normalizing heating 885, out of the furnace with compressed air to blow cold. Thickness of the seepage layer, hardness 1225 1600HV.

Boronization: boron infiltration heating 950, heat preservation for 4h, air cooling out of the furnace, permeability layer thickness, paste is 50% B4C + 50% Na3ALF6.

Liquid vanadium: heating 1000, heat preservation for 5h, air cooling out of the furnace. Seepage thickness. Hardness 2300 1800HV. Paste:.

It is feasible to use T8 quenching and tempering as a preparatory heat treatment. But the process of final heat treatment should be appropriate.

The preparatory heat treatment of T8 (A) generally does not do quenching and tempering treatment, since it is required to obtain spheroidized structure, then spheroidization annealing is good.

The shape is simple 770 heated, salt water cooled, 680 tempered. The complex shape is 800 heated oil cooled, 670 tempered.

Then I would like to ask the predecessor T8 how to spheroidize annealing to make the layer flake completely granular.

High-carbon steels are prone to burning carbon atoms in heat. In particular, the number of burns is too high or the temperature is too high.

The process of smelting cast iron into steel is largely a process of oxygen blowing and decarburization. It's easy to reduce the carbon content of 4% to even less.

As for the cracks, it may be caused by factors such as slag inclusion in forging.