This invention relates to achieving both superior Bending fatigue strength and Pitting fatigue life of gear(s) and/or shaft components, using "conventional alloy steel" by a particular method.
Ups-5,019,182 claims to use heat treatment route which docs not address Tempering process. In absence of tempering process after quenching, quenching stresses are not relieved prior to service leading to dimensional instability and susceptibility to - cracking. Further, the bending fatigue strength is not addressed in the claim.
In light of the existing prior-art, there is long standing demand to provide both superior bending and pitting fatigue life on gear(s) and/or shaft components simultaneously using "conventional alloy steel " which is described as cheaper, most widely used and widely available steels for gear(s) and/or shaft components. All above disclosures do not provide complete solutions for producing both superior bending fatigue strength and pitting fatigue life simultaneously.
It is an object of the present invention to provide solutions to achieve both superior pitting and bending fatigue strengths of gear(s) and/or shaft components
According to the present invention, this object is performed by a method according to claim 1. This method allows the use of "conventional alloy steel" (hereafter called only "conventional steel") which is described as cheaper, most widely used and most widely available for gear(s) and/or shaft components. The method includes the following steps in sequence.
The present invention allows to enhance the load carrying capability of transmissions without geometrical changes resulting in reduction of weights for higher load carrying capability, fuel consumption, development cost, development time and product cost and in turn give higher satisfaction to the customer.
The present invention allows to avoid geometrical changes in transmission components resulting in maintaining same weight and hence lower emission levels for enhanced load carrying capabilities.
The present invention allows to provide a solution to the problem of providing additional space in transmissions in case of geometric design changes are required to be introduced. The invention is also beneficial in such cases where the space constraints do not permit any geometric changes.
The unique figure shows heat treatment cycle of Modified Carbonitriding. This is followed by Hard shot peening
The present invention features achieving both superior bending fatigue strength and pitting fatigue life of gear(s) and / or shaft components using "conventional steel" by a method having following steps in sequence.
"Conventional steel" used in the present invention is either one of the following types.
Steel material comprising 0.10 to 0.30 weight % Carbon, 0.15 to 0.35 weight % Silicon, 0.8 to 1.5 weight % Chromium , 0.6 to 1.5 weight % Manganese , 0.017 to 0.040 weight % Aluminium , and balance iron including impurities , produced in vacuum degassing and alike routes.
Steel material comprising 0.10 to 0.30 weight % Carbon , 0.15 to 0.35 weight % Silicon , 0.3 to 1.5 weight % Chromium , 0.30 to 2.0 weight % Nickel , 0.08 to 0.50 weight % Molybdenum , 0.6 to 1.5 weight % Manganese, 0.017 to 0.040 weight % Aluminium and balance iron including impurities , produced in vacuum degassing and alike routes ,
The rationale for choosing the "conventional steel" having the said compositions are as follows:
Carbon inherently present in any steel is restricted in the range of 0.1 to 0.3 weight %. Lower than 0.1 weight % will not have sufficient core strength after the present processing. More than 0.3% will lead to core brittleness and reduced toughness. The response to heat treatment process will also be poor depending on higher Carbon contents.
Silicon is an essential element for de-oxidation of molten steel and hence minimum of 0.15 weight % is specified to ensure that de-oxidation is effectively taken care of. Higher than 0.35 weight % will entail more silicate inclusions affecting forgeability, machinability and reliability in service.
Chromium is easily available element for increasing hardenability. It is limited between 0.8 to 1.5 weight % to ensure adequate hardenability in the steels for gear(s) and / or shall components, in combination with Manganese. Higher than the limits will entail intergranular oxidation in the heat treated layers during carburising.
Manganese is yet another essential element effective in de-oxidation during melting and imparting hardenability. Not less than 0.6 weight % ensures de-oxidation and holds sulphur together. More than 1.5 weight % will lead to forgeability and machinability problems. It is easily available and cheaper element to increase the hardenability of the material for adequate core strengths and reasonable toughness Aluminium content in the range 0.017 to 0.040 weight % gives fully killed steel and does not contribute significantly in the nitride formation and stabilizing retained Austenite necessitating use of Modified Carbonitriding treatment for the purpose.
Trace elements like Nb , Ti , Zr , Cu and B are adjusted in such a way that the total contents are below 0.60 weight % . Nitrogen content is kept at 55 to 90 parts per million( ppm ) and hydrogen is not more than than 2.5 ppm . Calcium and Sulphur are usually added in suitable quantities to improve morphology of inclusions to facilitate machinability.
The steel during melting is treated by standard Vacuum degassing cycle to maintain lower oxygen contents (Oxygen content in the product not more than 20 ppm ) and hence limit size and distribution of inclusions to a degree that the component is fit for the applications already mentioned .
Carbon inherently present in any steel is restricted in the range of 0.1 to 0.3 weight %. Lower than 0.1 weight % will not have sufficient core strength after the present processing. More than 0.3% will lead to core brittleness and reduced toughness. The response to heat treatment process will also be poor depending on higher Carbon contents.
Silicon is an essential element for de-oxidation of molten steel and hence minimum of 0.15 weight % is specified to ensure that dc-oxidition is effectively taken care of. Higher than 0.35 weight % will entail more silicate inclusions affecting forgeability, machinability and reliability in service.
Chromium is easily available element for increasing hardenability. It is limited between 0.3 to 1.5 weight % to ensure adequate hardenability in the steels for gear(s) and / or shaft components, in combination with Manganese , Nickel and Molybdenum, of suitable quantities mentioned above. Higher than the limits will entail intergranular oxidation in the heat treated layers during carburising.
Nickel is another essential element effective in ensuring hardenability and improve toughness , required in critical applications. The required quantity is to be not less than 0.3 weight % for ensuring the toughness and hardenability. The upper limit is set to 2 weight % arrived at based on the effect in combination with other elements mentioned above.
Molybdenum is yet another highly effective element in promoting hardenability of the surface and in the core portion. The lower limit is set to 0.08 weight % to be effective in promoting hardenability. The upper limit of 0.5 % is set in combination with other elements mentioned above.
Manganese is yet another essential element effective in imparting hardenability, de-oxidation during melting .Not less than 0.6 weight % ensures de-oxidation and holds sulphur together. More than 1.5 weight % will lead to forgeability and machinability problems. It is also easily available and cheaper element to increase the hardenability of the material for adequate core strengths and reasonable toughness.
Aluminium content in the range 0.017 to 0.040 weight % gives fully killed steel and does not contribute significantly in the nitride formation and stabilizing retained Austenite necessitating use of Modified Carbonitriding treatment for the purpose.
Trace elements like Nb , Ti , Zr , Cu and B are adjusted in such a way that the total contents are below 0.60 weight % . Nitrogen content is kept at 55 to 90 parts per million( ppm ) and hydrogen is not more than than 2.5 ppm . Calcium and Sulphur are usually added in suitable quantities to improve morphology of inclusions to facilitate machinability.
The steel during melting is treated by standard Vacuum degassing cycle to maintain lowcr oxygen contents (Oxygen content in the product not more than 20 ppm) and hence limit size and distribution of inclusions to a degree that the component is fit for the applications already mentioned.
The gear(s) and/or shaft components are manufactured as per conventional gear machining practice for highway, off-highway vehicle transmissions and similar industrial transmissions. The said components after machining are loaded in a standard sealed quench furnace having requisite facilities for automatic measurement and feedback mechanisms for carbon potential, temperature and time and facility for ammonia introduction is to be in place. Furnaces other than standard sealed quench furnaces having above requisite capabilities are also covered in the object of the invention.
The first step in the heat treatment cycle is Carburising (Refer FIGURE). The carburising is done at 915 degree Centigrade with equal boost and diffusion periods with Carbon potential (Cp) 1.0 and 0.8 respectively , using carrier gas and enricher gases. The temperature of not less than 900 degree Centigrade at which the carbon diffusion is more pronounced is covered in the invention. The effective case depth covered is in the range of 0.3 to 1.7 mm (cut off hardness 513 Hv). Effective case depths less than 0.3 mm do not provide adequate pitting resistance and more than 1.7 mm have deleterious effects on the fatigue properties for the applications covered in the scope of invention.
At the end of carburising cycle , the component is cooled inside the furnace to 850 degree Centigrade and ammonia is introduced with 15% of the whole furnace gas mixture ( rest of the percent being carrier gas ). The cycle is carried out for minimum 30 minutes. Temperature not less than 840 degree Centigrade and not more than 870 degree Centigrade is also covered as part of the invention to facilitate pronounced nitrogen diffusion upto a depth of 0.3 mm. Similarly ammonia not less than 15% and not more than 20% of the whole furnace gas mixture is covered for the "conventional steel" in which nitrogen absorbing elements and elements promoting diffusion of nitrogen are not in sufficient quantities.
To minimize distortions in the steel components, quenching in suitable medium at 120 to 150 degree Centigrade is maintained in the present invention. Depending on the criticality of the component, the quenching medium temperature of not less than 50 C is covered in the object of the invention.
Tempering temperature of 180 degree Centigrade is adopted for the purpose of relieving quenching stresses, without reduction in retained austenite produced after quenching, as above. The temperature not less than 160 degree Centigrade is covered to relieve quenching stresses.
Hardness after Modified Carbonitriding is maintained at not less than 74U Hv at a depth of 0.05 to 0.35 mm below the surface. The stresses responsible for pitting (called "Hertzian" stresses) are maximum at depth range mentioned here in the applications mentioned above. The hardness will get further enhanced during Hard shot peening and will provide adequate safety against pitting failures for the applications already covered.
The bending fatigue strength, which is a function of maximum residual compressive stress below the surface, is also enhanced by Hard shot peening.
Further processing by Hard shot peening of the gear(s) and/or shaft components has simultaneous benefits of increasing the bending fatigue strength not less than 30% and pitting fatigue life by more than 3 times. The results have been confirmed in severe, rigorous and accelerated transmission endurance trials for life time, in comparison with conventional " carburising " component run with conventional monograde GL-4 gear oil , with oil temperature reaching upto 95 degree Centigrade. Similar results are covered with GL-4 or higher performance category multigrade oils with the present invention.
The improvement in bending fatigue strength results are further confirmed with Residual stress measurements using non- destructive Rigaku X-ray diffraction treatment upto a depth of 150 microns of actual component with conventional "Carburising " route and " Modified Carbonitriding with Hard Shot Peening " method using "conventional steel" . The maximum residual compressive stresses of 1500 Mpa and corresponding bending fatigue strength improvement of 30 to 80 % are covered in the present invention.
The roughness and finish of the component surface influences the lubrication condition during engagement of with the mating components. Keeping in mind that the gears need to be within the intended surface quality norms, the parameters are limited to as below,
shot size ranging from 0.5 to 0.8 mm,
shot hardness 610 to 800 Hv,
shot velocity 60 to 150 m/sec ,
part coverage 200 to 500 % Almen A arc height 0.6 to 0.9 mm.
This list of references cited by the applicant is for the reader's convenience only. It does not form part of the European patent document. Even though great care has been taken in compiling the references, errors or omissions cannot be excluded and the EPO disclaims all liability in this regard.