Patent Application
20070246126
Hereinafter, tests conducted in order to confirm the effects of the present invention will be explained.
First, steel having a chemical composition shown in Table 1 of 150 kg was molten in a high-frequency vacuum induction furnace. The obtained steel ingot was rolled or hot-forged to be a round bar having a diameter of 90 mm, and further hot-forged to be a round bar having a diameter of 22 mm to 32 mm, both ends inclusive, as required, so as to obtain a test material.
The obtained test materials were evaluated as follows:
Manufacturability was evaluated by evaluating hardness after annealing. That was, a normalizing process at 920° C. for one hour is applied to a round bar test piece having a diameter of 32 mm and a length of 100 mm, and further a annealing process at 760° C. for five hours was applied. The obtained test piece was measured at a position of half radius of the cross-section (cross-section perpendicular to the axis) with Rockwell Hardness B-Scale, HRB, according to JIS:Z2245, and HRB 90 or less was determined as excellent machinability.
Round bar test pieces having a length of 100 mm were produced respectively as carburizability test pieces from forged steel bars having diameters of 10 mm and 20 mm. The carburizing process used a vacuum carburizing furnace, and acethylene as carburizing gas, and by controlling a propane gas flow rate, carburizing diffusion time and a carburizing temperature, the surface C concentration was controlled within a range of 1.15% by mass to 4.01% by mass, both ends inclusive. Additionally, the carburizing conditions were as follows:
The first carburizing process: In order to have a top surface C concentration of approximately 1.0% by mass, after applying a carburizing process at 1100° C. for seventy minutes, the test piece was rapidly quenched by gas cooling to a temperature range of 500° C. or less, so as to infiltrate C into the steel to a high concentration range that carbide did not precipitate.
The second carburizing process: According to a target carburizing concentration, after applying a precipitation process by retaining a temperature range of 850° C. to 900° C., both ends inclusive, and further according to a target C concentration, a carburizing process was conducted in a temperature range of 850° C. to 900° C., both ends inclusive, for 60 to 120 minutes, both ends inclusive, and then a quenching process was conducted in an oil tank of 130° C. Also, after the quenching process, a tempering process was conducted at 180° C. for 120 minutes. Additionally, for the test piece No. 21, a steel ball having a diameter of 0.6 mm and hardness of 700 Hv was used, and after carburizing, shot-peening was applied under a condition having a coverage of 300% and a arc height of 0.5 mmA.
Hereinafter, items for which the evaluation was conducted will be explained. The evaluation results are shown in Table 2.
Surface C concentration: After the carburizing process, a C concentration at 25 μm deep from a surface of the processed test piece was measured by EPMA (Electron Probe Microanalysis) combined with SEM.
Carbide area ratio: A cross-section of the round bar test piece, which had been carburized, quenched and tempered, was polished and corroded with picral etchant, and then a photography thereof was taken at 25 μm deep from the top surface by SEM (observation magnification of 3000×), and by image analysis the area ratio was measured.
Carbide size: The area ration of carbide having 10 μm or less was measured by observing the same conditions as above.
Presence of cancellous carbide: Presence of cancellous carbide was examined by observing the same conditions above.
Presence of imperfectly-quenched structure: A cross-section of the round bar test piece, which had carburized, quenched and tempered, was polished, corroded with nital etchant, and then observed at 25 μm deep from the top surface with an optical microscope, so as that presence of an imperfectly-quenched structure was examined.
Depth of grain boundary oxidized layer: A cross-section of the round bar test piece, which had carburized, quenched and tempered, was polished, and then observed with an optical microscope in a uncorroded state, so as that depth of a blackish layer along the grain boundary of the top surface was measured.
Temper softening resistance: The round bar test piece, which had carburized, quenched and tempered, was further tempered at 300° C. for 180 min., polished, and then measured at 25 μm deep from the top surface with Vickers Hardness (test weight: 200 g)Hv according to a method specified in JIS:Z2244, also determining that the strength improving effect was sufficient in a case Hv750 or more was obtained (The strength was improved 30% or more comparing with a gas eutectoid carburized piece composed of SCR420 material).
Identification of carbide volume ratio: It was conducted by measuring an X-ray diffraction profile as explained above. The above tests were conducted respectively using a test piece having a diameter of 10 mm.
Non-carburized layer strength: Using a test piece of a diameter of 20 mm, a cross sectional center portion of the test piece was measured with Rockwell Hardness C-scale, HRC, and the non-carburized layer strength having HRC 30 or more is determined as accepted.
Surface fatigue strength evaluation: A fatigue test was conducted by a widely-known roller pitching testing machine, and load surface pressure causing no roller pitching at 107 cycle was defined as the surface fatigue strength, so as to conduct the evaluation. Specifically, a round bar having a diameter of 32 mm was first heated and retained at 950° C., and then slowly-cooled to be softened, and a roller pitching test piece having a test part diameter of 26 mm was fabricated by machining. Also, ball-bearing steel (SUJ2) was used as a material of an opposing roller to the test piece, and quenching and tempering processes were applied so as to have a hardness of HRC61. Additionally, a curvature radius of the big roller was 150R or 700R. A carburizing process to the test piece was conducted simultaneously with the carburizing process conducted in order to conduct the above-described basic evaluation test of the inventive steel. Additionally, after the carburizing process, a part of the pitching test piece was tempered by retained at 300° C. for 3 hours, and then evaluated with surface C concentration, carbide area ratio, maximum carbide size and tempering hardness. Also, the surface fatigue strength set surface fatigue strength of the gas eutectoid carburized piece specified in JIS:SCR420 as a reference value (1.0), each material strength was shown with a magnification index to the reference value, and in a case of achieving the surface fatigue strength of 30% or more than the reference value, it was determined that the strength improving effect was sufficient. The above results are shown in Table 2.
According to the above results, it can be recognized that the respective example pieces do not show imperfectly-quenched structures, cancellous carbide, or grain boundary oxidization, which cause strength deterioration, and have excellent manufacturability (annealing hardness≦HRB90), can obtain tempering hardness (≧750 Hv) at 300° C. sufficiently, and have excellent fatigue strength.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2006-116308 | Apr 2006 | JP | national |
| 2007-035632 | Feb 2007 | JP | national |
