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Chemical composition analysis is done by following methods: |
 Spectro Vac Analysis - Spectro vac can analyse upto 12 elements.
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| Wet Analysis.
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Spectro. |
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Carbon and Sulphur Analysis. |
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Titration. |
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Macro Structural Examination |
Defects like inhomogeneous structure, segregation, shinkage / gas porasity, flute cracks, flakes and splash etc. can be studied.
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Micro Structural Examination |
Structure analysis of ferrous and non-ferrous metals are carried out under Optical microscope at magnifications 100x, 400x and up to 1000x as detailed below. |
Rate Of Inclusion: Type of inclusion and distribution as per ASTM chart E-45. |
Average Grain Size: Austenite grain size of steels comparable to ASTM chart E-112 (by Mc QUID - EHN Process) |
Structure: Observed at higher magnifications to study the phases in structure before and after heat treatment to assess the structure is satisfactory or detrimental to the properties of the material. |
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Vicker's Hardness Test |
DIGITAL DISPLAY MICRO HARDNESS TESTER. HVS-1000: VERSATILE Features like measuring surface hardening, case depth measured by hardness servey by using smaller loads in the range of 10gms - 1000gms.
Measuring hardness of surface plating / coating by selecting smaller loads depending upon the depth of plating/coating thickness. |
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Jominy Hardenability |
Hardenibility is defined as the susceptibility of the steel to hardening when quenched from above the upper critical temperature and is related to the depth and distribution of hardness across the cross section. Hardenibility is measured for specific hardness or specific percentage of martensite after heat treatment and end quench/ cooling slowly by using the jominy end quenching equipment. |
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Back Ground |
Premature failure of the component of material specification EN 36C was forwarded to Hi-Tech Laboratory for investigation to ascertain the mode and cause of failure (ref fig 1). |
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Fig 1. Broken Part |
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Observations |
Visual / Macro Examination |
The part was broken squarely, observed severe rubbing, heat tint along the axis. On macro examination the rubbing areas showed numerous parallel cracks (ref fig 2)
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Fig 2. Enlarged view of Fig. 1 showing rubbing and heat tint with parallel cracks shown by arrows |
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Fractographic Examination |
The cleaned fracture surface is observed under stereomicroscope at 10x and 40x magnifications. The fracture surface revealed beach marks on both the opposite sides. The orientation of beach marks indicates the fracture initiation is from the outer surface (ref fig 3 and 3a).
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Fig 3. Beach Marks an either side of step, shown by arrows. |
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Fig 3a. Enlarged view of Fig 3. |
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Chemical Analysis by Spectro Vacc |
The chemical analysis was found confirming to EN 36C. |
Vickers Micro hardness testing |
The case depth measured as 1.9mm. The case hardness at the surface measured as 50-51 HRC, Core hardness as 36-38 HRC.
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Micro Examination |
The cut section of the fracture was polished and etched with natal. The etched surface observed under microscope at 500x magnification revealed martensite structure consists of retained austenite (30 to 40%) fig 5 & Fig 6 and core revealed bainite structure.
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Fig 4. showing retained austinite (white), at 100X. |
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Fig 4a. showing Martenisite-Dark, Retined Autinite - White, at 500X. |
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Conclusion
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Based on the above observations and analysis the chemistry of the metal was found meeting the specifications of EN 36C, the case structure and hardness is not meeting the requirement due to retained austenite and low hardness. The core revealed Bainite structure and low hardness.
The failure is attributed to fatigue. The fatigue crack initiated on outer surface in the presence of retained austenite. The fatigue observed on opposite sides possibly due to deflection of component during performance. The cracks developed on the surface are due to severe rubbing associated with higher retained austenite (30-40%) and residual tensile stresses.
Further the crack propagated by vibrations/cyclic stresses leading to fatigue failure as evidenced by the orientation of beach marks on either side of the fractured zone.
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Recommendations
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The recurring of such failure can be avoided by |
| Controlling the heat treatment process and parameters.
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| Design can optimize to eliminate the assembly errors.
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Tensile Testing
Electronic Universal Testing machine is of 40 Ton capacity & having rigid construction. Modern electronics in hydraulic machine have features higher productivity & it offers many facilities useful in testing various materials. The UTE type universal testing machine serves conducting tests in metals like Tension, Compression, and Traverse.
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Impact Testing
FIE's universal pendulum impact testing machine IT-30 serves for conducting impact tests. On this machine Charpy & Izod tests according to IS: 1757-1973, IS-1499-1977 & IS 1598-1960 Standards will be carried out to determine the behavior of metals, especially of Steel & Steel casting under impact stresses. These tests enable the supervision of heat treatment process & give the proof of susceptibility to brittle fracture.
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Ultrasonic Testing
Ultrasonic testing uses sound waves of short wave length & high frequency to detect flaws in casting, forgings, steel plates, machined components, rolled bars, weldments, pressure vessels, mill components etc. Tests will be carried out by level II qualified Engineer.
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Magnetic Particle Testing
Magnetic Particle Testing (MPT, yoke type) is carried out to detect surface and sub-surface flaws in ferromagnetic materials such as steel & Iron. Fluorescent wet /dry MPT method will be performed by Level II qualified Engineer.
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Liquid Penetrant Testing (PT)
Liquid Penetrant Testing is a method that is used to reveal surface flaws by bleed out of a coloured or fluorescent dye from the flaws. Certified Penetrant Chemicals conforming to national & international specifications will be used for reliable testing. This test is performed by Level II qualified Engineer.
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