High-speed Steel High-precision Circular Knives Heat Treatment Process

In order to meet the needs of the international cutting tool market, Nanjing Jingfeng Cutting Tool Company has developed HSS steel round knives, with an outer diameter of 200∼400mm, a thickness of 5∼10mm, and an inner hole of 145∼254mm. The round blade requires a hardness of 62.5∼64HRC; flatness and parallelism ≤0.002mm; straight angle ≤0.005mm. The φ310mm×5mm×180mm blade is shown in Figure 1. This knife is rarely used as a single piece, but mostly in groups. For example, 30 pieces are assembled on a CNC machine tool and cut at the same time, so the accuracy requirements are very high. The circular knife salt bath quenching process is introduced as follows.

Figure 1Workpiece indication

Annealing after forging

A batch of blade materials must have the same heat number to ensure the consistency and stability of the heat treatment process. It should be annealed in time after forging. The process is (850∼860℃)×(3∼3.5h), the furnace is cooled to 750°C and isothermal: (730∼740℃)×(3∼4h), the furnace is cooled to 500°C, and the furnace is air-cooled. Do not open the lid, wait until the workpiece is cold below 300℃, then open the lid to slow down. The hardness after annealing is ≤255HBW. The annealing process should be strict to prevent problems such as oxidative decarburization and carbide stabilization. If the annealing quality is not good, it will not only affect the heat treatment quality but also affect the corporate reputation.

Salt bath quenching process

(1) Wire tie

Use 5-6 strands of 22-gauge fine iron wire for binding, but do not use thick iron wire. Because the thin iron wire has good plasticity, it clings to the surface of the blade under high temperature heating. During heating and subsequent entering and exiting the furnace, the blade does not swing, it is in surface contact, and the single thick iron wire is in point contact with the blade. When the round knife is lifted and put down, it will definitely shake and increase its deformation.

(2) Stress relief

It is carried out in a pit furnace, and the process is: (450∼500℃)×4h.

(3) Warm-up

The following are examples of φ310mm×5mm×180mm (inner hole). The preheating process is 900℃×6min40s, each string is hanging 3 pieces, two strings are placed in the furnace, the formula of the medium temperature salt bath (mass fraction): 70%BaCl2+30%NaCl.

(4) Heating

The high temperature salt bath is 100% BaCl2, the heating temperature is 1195~1200℃, the heating time is 3min20s, and a string is placed in the furnace.

(5) Cooling

First, it is classified in neutral salt at 500∼560°C and then classified in nitrate at 240∼280°C for the second time. The formula of the neutral salt bath is 50%CaCl2+30%BaCl2+20%NaCl; the formula of the isothermal nitrate is: 50%KNO3+50%NaNO2, the metallographic control in front of the furnace is 11~10, and the carbide dissolves generally That's it. The quenching metallography of the M2 steel circular knives is shown in Figure 2. The hardness after quenching is 64∼64.5HRC.

Figure 2 Quenching metallographic control level

(6) Flattening

When the workpiece is cooled to 240∼280℃, it will come out of the oven, immediately cut off the iron wire, remove the salt stain, and flatten it with a hydraulic press.

(7) Cleaning

The circular knives that have been cooled to room temperature are removed from the hydraulic press and washed in hot water, rinsed with cold water and dried.

(8) Pre-tempering

Clamp the cleaned and dried blades straight. Do not tighten the screws too tightly. Temper (380∼400℃)×(3∼4h) in a pit furnace. Tighten the nut immediately after it is out of the furnace.

(9) Tempering

550℃×1.5h×3 times nitrate salt tempering, the hardness after tempering is 65～65.5HRC.

(10) Adjust the hardness

The hardness after tempering at 580℃×1.5h×1 time is 63∼64HRC.

(11) Inspection

Check the curvature, sampling hardness and metallography one by one. Due to the same furnace number and strict process discipline, the final inspection pass rate reaches 100%.

(12) Final stress relief

Since the precision of the finished product is very high, the amount of fine grinding is 0.10∼0.15mm after rough grinding, and the stress is relieved in the air furnace: 260℃×4h.

For M2 steel, if heating and quenching at 1175∼1180°C and tempering at 550°C×1.5h×3 times, the acceptable hardness of 62.5∼64HRC can also be obtained. However, practice has proved that quenching at 1195∼1200°C and tempering at 550°C ~ +580°C are good, both have the same hardness and curvature, but the latter has high wear resistance and long life.