Industrial process analysis and control of wear-resistant Bainite Steel Ball

The grinding ball is the easy-consumption part of the ball mill. The high-quality grinding ball can reduce the overall production cost, improve the overall economic efficiency, reduce the number of downtimes, improve the utilization of ball mill, reduce labor costs. MN-B alloy bainite steel ball has the characteristics of low cost, high strength, and excellent wear resistance. Its yield strength is more than 1000MPA, and it is suitable for industrial production.

  1. There’s a quality issue

Although the Bainite Steel Ball has passed the conventional quality inspection in batch sampling inspection, including Bainite metallographic examination, impact strength resistance test (≥10kgm / CM2), drop ball test, etc.  However, in the storage of 1 ~ 3 months, it will happen naturally cracking phenomenon. The following are some of the processes in which cracking occurs:

Rolling stock- cutting stock – heating – forging – strong wind quenching into cooling bed, cooling to a certain temperature-lower BAINITE ISOTHERMAL transformation – holding – tempering – sampling inspection;

The casting blank is pressurized continuous rolling-heating-forging-water cooling bed quenching, cooling to a certain temperature-lower BAINITE ISOTHERMAL TRANSFORMATION-HOLDING-TEMPERING-SAMPLING INSPECTION

Steel Ball cast in metal mold-water cooling bed, water quenching, cooling to a certain temperature-lower BAINITE ISOTHERMAL transformation-holding-tempering-sampling inspection.

  1. Metallographic structure of Bainite and cause analysis of Heat Treatment

BAINITE steel ball is a kind of MN-B metallographic structure; its most prominent characteristic is that it is easy to produce thermal stress and form Brittle crack in the process of cooling quenching and low temperature tempering cooling.

The causes of cracking are analyzed as follows: (1) The cooling is too fast in the process of cooling and quenching, and the temperature difference between surface and interior is too great.

When the surface layer has cooled down, the deep part of the heart is still in a high-temperature state; the outer surface begins to shrink to a little change, the cooling contraction of the heart increases, resulting in a triangular crack;

The same phenomenon also occurs when cast steel balls are used. Through the experimental analysis, when the cast steel balls are cast by pressure casting in a casting concatenated rolling mill at the temperature of 1300 ~ 1000 °C, the internal thermal stress in the casting is eliminated and the triangle crack in the deep part of the core is restrained Air Cooling speed is too fast, resulting in thermal stress, causing natural cracking, so after tempering, buried sand thermal insulation treatment, Slow Cooling, eliminate tempering internal thermal stress.

  1. Improvement plan of industrial production process

Improvement Option One

Steel blank cutting (or casting continuous rolling pressurized blank)-heating-forging-cooling bed breeze (or water mist) quenching cooling to (control cooling rate 20 ~ 45 °C / Min) a certain temperature-lower Bainite isothermal transformation about 25 min-air cooling to 280 ~ 220 °c-pit heat preservation-tempering (attention to control temperature and time)-buried sand heat preservation cooling-test and so on.

Improvement option Two 

In Metal Mould Casting, the quenching process is carried out by pressure-cooling bed, cooling by breeze (or water mist), cooling to (control cooling rate 20 ~ 45 °C / Min) , constant temperature-lower Bainite isothermal transformation about 25 min-air cooling to 280 ~ 220 °c-pit holding temperature-tempering (attention to control temperature and time)-sand-burying holding temperature-cooling-testing, etc. .

4. I mproved process inspection results:

The experiment of the metallographic structure shows that the bainite structure under needle shape is uniformly refined, as shown in Fig. 1. The samples were taken by JB-3DB impact Tester (specification 101055). Fig. 1 impact toughness ≥100J / CM2.

The hardness is 52 ~ 56HRC tested by KPE-3000 Brinell hardness tester.

Through the test of 3.5 m high simulation ball falling machine, 32 balls were randomly selected as a group, the impact times were more than 10,000 times, and the breakage rate was less than 0.5 %.

5. User effects 

By using 3.2 m 3.1 m ball mill to grind medium and high hardness gold-copper ore, 250 G / T (350ー500 G / T lower than usual) was consumed.