MILL SHAFTS

Abstract

The present invention belongs to the field of mechanical engineering and materials, more specifically in the metallurgy segments, for application in the sugar industry. The invention relates to the hot forging process of mill shafts and heat-treated auxiliary equipment in CrNiMo and low carbon alloy. The shafts and auxiliary equipment manufactured according to this invention have a long service life, thus reducing downtime for maintenance and increasing reliability, since it eliminates the problems associated with cracks and instantaneous fractures. As a consequence, there is a reduction in the risks of accidents and production losses associated with stoppages.

Claims

1. Low alloy steel based on Chromium-Nickel-Molybdenum, characterized in that it comprises the following chemical composition in mass percentage: 0.28 to 0.33% C; 0.50 to 0.60% Mn; 0.15 to 0.35% Si; 0.80 to 1.10% Cr; 0.15 to 0.25% Mo; 0.35 to 0.50% Ni; an amount of less than 0.025% of P; an amount of less than 0.025% of S; an amount of less than 0.35% of Cu; an amount of less than 0.35% of V; and a maximum of 3.0 PPM of H.

2. Low alloy steel based on Chromium-Nickel-Molybdenum, as in claim 1, wherein it is applicable to manufacture of gears, shafts, screws, beams, and machine parts.

3. Low alloy steel based on Chromium-Nickel-Molybdenum, as in claim 1, wherein it is applicable to manufacture shafts for sugar cane mills and auxiliary equipment.

4. Shafts for sugar cane mills characterized in that the mill shafts are manufactured according to the low alloy steel as described in claim 1.

5. Shafts for sugar cane mills, as in claim 4, wherein the mill shafts are hot forged, in a 2500 Ton press, with an area reduction ratio greater than 3:1.

6. Shafts for sugar cane mills, as in claim 4, wherein the mill shafts undergo a normalization treatment, with austenitization temperature of 860 to 890° C.

7. Shafts for sugar cane mills, as in claim 6, wherein the mill shafts undergo a normalization treatment, with an average heating speed of 80° C./h, in electric or natural gas furnaces, being maintained at austenitization temperature for a minimum period of 0.5 h/inch of shaft thickness.

8. Shafts for sugar cane mills, as in claim 7, wherein the mill shafts undergo a normalization treatment, being cooled to room temperature in still air.

9. Shafts for sugar cane mills, as in claim 6, wherein the mill shafts undergo a quenching treatment, with austenitization temperature of 870 to 910° C.

10. Shafts for sugar cane mills, as in claim 9, wherein the mill shafts undergo a quenching treatment, with an average heating speed of 80° C./h, in electric or natural gas furnaces, being kept at the austenitizing temperature for a minimum period of 0.5 h/inch of shaft thickness.

11. Shafts for sugar cane mills, as in claim 10, wherein the mill shafts undergo a quenching treatment, being cooled down to a temperature below 150° C., with cooling carried out by water in a tank with 130,000 liters and with agitation by pumps and compressed air.

12. Shafts for sugar cane mills, as in claim 11, wherein the mill shafts undergo a quenching treatment, with heating up to a temperature between 620 and 660° C.

13. Shafts for sugar cane mills, as in claim 12, wherein the mill shafts undergo a quenching treatment, with an average heating speed of 80° C./h, in electric or natural gas furnaces, being kept at the heating temperature for a minimum time of 0.75 h/inch of shaft thickness.

14. Shafts for sugar cane mills, as in claim 13, wherein the mill shafts undergo a quenching treatment, being cooled to room temperature in still air.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0017] The present invention refers to the forging of mill shafts and auxiliary equipment, for use in the sugar cane industry. The shafts are manufactured using a low carbon steel alloy based on Chromium-Nickel-Molybdenum, hot forged in a 2500 Ton press with an area reduction ratio greater than 3:1, heat treated (normalization, quenching and tempering) for hardness ranges between 200 and 250 HB and tensile strength greater than 620 Mpa, yield strength greater than 380 Mpa and impact strength greater than 80 J at room temperature.

[0018] The ACF130 alloy is specially modified to present high mechanical properties compared to the state of the art, increasing the reliability of the shafts in operation and reducing the necessary stops for predictive maintenance. The manufacturing process presented in the invention provides parts with high tensile strength and good hardness, reducing the occurrence of damage due to use, especially cracks and mechanical fatigue.

[0019] Alloy ACF130 presents the following chemical composition in mass percentage: 0.28 to 0.33% of C; 0.50 to 0.60% of Mn; 0.15 to 0.35% of Si; 0.80 to 1.10% of Cr; 0.15 to 0.25% of Mo; 0.35 to 0.50% of Ni; an amount of less than 0.025% of P; an amount of less than 0.025% of S; an amount of less than 0.35% of Cu; an amount of less than 0.35% of V; and a maximum of 3.0 PPM of H.

[0020] Hot forging with a reduction ratio greater than 3:1 ensures that the shafts are free from internal defects. Hot forging is carried out in a press with a capacity of 2500 Ton.

[0021] The normalization heat treatment aims to homogenize and refine the steel microstructure, in order to better receive the subsequent thermal treatments. Normalization is carried out by fully heating the shafts in an electric or natural gas furnace, at an average heating speed of 80° C./h, at a temperature higher than that of austenitization, between 860 and 890° C. The permanence time of the pieces in the furnace depends on the diameter of the shaft to be manufactured, with this permanence time in the furnace being 0.5 h/inch of shaft diameter. The shafts are cooled slowly by still air to room temperature.

[0022] The improvement of the shafts (heat treatment of quenching+tempering) aims to obtain sufficient tensile strength to support the workloads, guaranteeing optimized mechanical properties, meeting the solution of the problems identified in the state of the art. Quenching is carried out by heating the shafts to a temperature higher than the austenitizing temperature, between 870 and 910° C., in an electric or natural gas furnace, at an average heating rate of 80° C./h. The permanence time of the pieces in the furnace depends on the diameter of the forged shafts, with this permanence time in the furnace being 0.5 h/inch of shaft diameter. The shafts are cooled in a tank holding 130,000 liters of water and stirred by pumps and compressed air, to a temperature of less than 150° C. Tempering is carried out by heating the shafts to a temperature between 620 and 660° C., to meet the specified hardness range. Heating is carried out in an electric or natural gas oven, at an average heating speed of 80° C./h. The permanence time of the pieces in the oven depends on the diameter of the forged shafts, this time being 0.75 h/inch of shaft diameter. The cooling of the shafts after tempering is carried out by convection, in still air, to room temperature.