A new technique to estimate the brittleness of the case of carburized and
Fig.1 Shape of Debye ring and the run of residual stress Very near the surface, the X-ray irradiating spot is eclipsed by the edge of the specimen and the reliability of the measurement drops. The measuring reliability in the range of larger distance over 0.8mm is considered OK
Among mechanical engineers, there is a strong belief or piety: the higher the hardness of steel, the stronger it is. Many people concerning gears think therefore, the higher the gear tooth flank hardness, the more durable and tougher the gear. In reality though, excessively carburized and hardened gear often experiences failure in usage. We have to have a method to check, whether a heavily carburized and hardened gear is OK or liable to fail due to the embrittlement of the hardened layer.
Power transmission gears are usually case carburized and quenched to raise the tooth flank hardness for increasing load carrying capacity. The nature of materials of simple construction is in general, when the hardness increases, the ductility decreases and it becomes brittle. When we consider the hardened tooth flank, it is mainly of martensite that consists from many small long form crystals in the construction of lamellar layers of cementite and of ferrite. Such complex construction gives high average hardness with nottoo-fragile nature to gear tooth flank. Power transmission gears are usually demanded to have thick hardened case whose thickness is larger than the depth of the position of the maximum shearing stress in the subsurface induced during operation. It is generally believed that the high hardness increases material strength, and many gear people aim harder and harder steel for their product gears. To have such thick hardened case, gear must be heated for very long carburizing time under high temperature to achieve enough thermal diffusion of carbon into steel. In addition, it is often experienced that the distribution of alloy elements is not homogeneous inside high strength steel. To obtain proper strength of such steel, it is necessary to give strong plastic deformation through strong forging, but it is not well done recently to reduce production cost. As the consequence, the construction of the hardened carburized case deteriorates and the hard case becomes fragile, though the average hardness is kept high and looks normal. To escape from the trouble, case carburized and quenched gear is then annealed, but whether it is done correctly or not is usually not checked. It is necessary to check, whether the hardened case of the tooth does not have brittle nature.
Fig. 2 Comparison of micro-Vickers’ hardness and FWHM distribution
Fig.1 shows an example of the measured residual stress and Debye rings of X-ray diffraction for a case of failed tooth flank of big bevel gear for naval use. It is worth mentioning that the 3D form of Debye ring is not of axially symmetric smooth crator, but whose ridge peak hight varies along 360 deg. cercumference. This state means that the orientation of crystals of this steel is biased. At some positions, considerable residual tensile stress is observed, that has surely big connections with the cause of the failure. It is well known, that the FWHM (full width at half maximum of the Debye ring) has a strong positive correlation with hardness. When we compare the behavior of micro-HV distribution in depth direction with the behavior of the FWHM, we find a contradiction from our such common knowledge mentioned above: When we see the run of both the distributions of FWHM and micro-HV near the surface of the failed tooth flank, cf.Fig.2, the FWHM drops toward the surface, but the microHV distribution does not. The meaning of this discrepancy can be explained as shown in Fig.3: At the HV measurement, the stylus makes indentation on the target surface. During this action, the stylus induces plastic deformation of the mixture of hard inclusions and precipitation particles together with the steel matrix. The measurement of indentation making HV reflects therefore the degree of the resistance force to make such plastic deformation of the mixture of hard particles and rather softer bonding matrix that is mainly consist from ferrite.
Fig.3 Contribution of ferrite matrix and hard particles like cementite on X-ray diffraction and Vickers’ hardness
On the other hand, the X-ray diffraction measures the microscopic deformation of ferrite crystals existing in ca. 8mm depth from the surface. Cementite and other hard particles are free from X-ray diffraction for making Debye ring. When good steel is well carburized and quench-hardened, hard particles occurr and push the surrounding matrix inside the steel material and compressive residual stress appears in the ferrite there and the FWHM increases. The FWHM of X-ray diffraction in such a case can be the index for the hardness or for the strength of the material that includes total matrix and hard particles. On contrary, the state of increasing HV with decreasing FWHM means that the hardness of the matrix of the steel decreases, but the existence of hard particles resists the plastic deformation of the total steel to keep the macroscopic hardness. The degree of the lack of ductility corresponds to the ratio between increasing HV value and the degree of FWHM decreasing. The measurement of case carburized and quenchd steel with X-ray deffraction and simultaneous indentation making micro-HV with light pressing load can reveal the state of embrittlement of the surface of the treated steel. For further information, see the report “Estimation of brittleness of CCH-steel via measurement of micro-VH and simultaneous FWHM distribution”, that will be published at the coming 6th VDI International Conferenc on Gear Production 2025, September 10-12, at FZG, Garching/Munich, Germany.
That paper deals with the following issue:
- Actual example of tooth flank failure of such mode experienced with big naval bevel gear
- Definition of effective tooth flank form deviation
- FWHM drops while the hardness keeps harder value
- Tensile stress can appear in the case of heavily carburized and hardened gear teeth.
About the Author:
The author, Aizoh Kubo, Prof.h.c.Dr.-Ing., is a professor emeritus of the Kyoto University, Japan, and now the president of the Research Institute for Applied Sciences. This institute RIAS is a foundation authorized by the Japanese government for public benefits and interest and is the only one related to mechanical engineering.