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ML00081 - Aug.2017

Hydrogen Cracking of a Chromium Plated Carburized Pin

Micrography of a longitudinal section of the pin, in the center. The low carbon content of the steel in the center of the pin makes the structure bainitic even though it was quenched and tempered after carburizing.

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of a of the , in the center. The low carbon content of the in the center of the makes the bainitic even though it was after carburizing.
M

Material

Low Alloy for Thermochemical Treatment
  • AISI 8620
Ds

Description

An , that was operating in a hydraulic system, has suffered rupture by after less than one year in service. The investigation showed that the pin was before the , and there were several probably formed during the electrolytic plating due to a effect of a too high hardness . These preexistent easily propagated by fatigue in a region that there was a lubrification hole.

C

Chemistry (% in weight)

Analysis in the center

0.21C; 0.21Si; 0.80Mn; 0.026P; 0.017S; 0.49Cr; 0.42Ni; 0.18Mo; 0.08Cu.

Associated Images
Scanning electron fractography of a chromium plated pin, close to the pin surface where the fatigue cracking started, showing intergranular morphology typical of a hydrogen embrittlement effect. It occurred because the low alloy pin was carburized, quenched and tempered, with a too high hardness structure, before the electrolytic plating.
837
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Brittle Fracture
Aspect of three pins that were operating in a hydraulic system. Adjacent to the fracture of the broken pin (the one that is in the right side of the figure) it is possible to see multiple surface craks and detachment of the surface chromium layer.
88
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Aspect Before AnalysisDf - General View
The aspect of the pin fracture surface shows that a fatigue crack propagated from the pin surface at the edge of a lubrication hole in the chromium plated pin.
83
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Aspect Before AnalysisDf - Fatigue Fracture
Micrography of a longitudinal section of the pin, in the center. The low carbon content of the steel in the center of the pin makes the structure bainitic even though it was quenched and tempered after carburizing.
80
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - In the Center of the Thickness
Micrographic detail of the bainitic structure in the center of the pin, that was quenched and tempered after carburizing.
75
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Bainite
Micrography of a longitudinal section of the pin, at the surface. The pin was carburized, quenched and tempered (yellow diffuse layer at the surface, 0.5 mm of extension) and a white layer of hard chromium plating is evident.
108
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Carburized Layer
Micrographic detail a longitudinal section of the pin, at the surface, showing the case hardened. The pin carburized, quenched and tempered structure presents a gradient of carbon content and consequently a gradient of martensitic structure and hardness. Close to the surface the hardness level is 742 HV1 .
87
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - At the Surface
Micrographic detail of the high hardness martensitic structure at the surface of the pin, that was quenched and tempered after carburizing.
67
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Martensite
Macrography of the longitudinal section of the 2" diameter pin close to the fracture surface (top of the figure) in a section where the pin presented the final fracture (it is deformed). The lubrication hole is in the right side of the figure. It is possible to see several secondary cracks at the surface of the pin (left side of the figure), propagating only where there is a carburized layer. It is possible to see that the end of each secondary crack is blunted.
67
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - NItal Etched MacroDf - Carburized Layer
Micrography that presents a secondary cracking (adjacent to the broken surface) propagating 500 microns from the surface, where there is a white layer of hard chromium .
78
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Carburized Layer
Micrographic detail of an intergranular secondary crack (adjacent to the broken surface) propagating from the hard chromium layer.
75
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Nital Etched Micrography by OMDf - Intergranular Cracking
The as polished micrography of a longitudinal section of the pin, showing several microcracks associated to the chromium layer.
56
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Micrography by OMDf - Hard Chromium Layer
The as polished micrography of a longitudinal section of the pin, showing that under the white layer of hard chromium (about 10 microns thick), there are already intergranular microcracks.
52
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Micrography by OMDf - Hard Chromium Layer
Scanning electron fractography showing that the fracture micromechanism is intergranular close to the surface of the pin, suggesting a hydrogen embrittlement effect at the carburized layer. The cracks were created during the electrolytic plating, and propagated later during service.
45
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - At the Surface
Scanning electron fractography of the pin in the fatigue subcritical crack growth, showing striation and regions of smashing .
40
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Fatigue Striation
Fractographic detail of the fatigue striation in the subcritical crack growth.
37
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Fatigue Striation
Fractography of the final fracture of the pin, presenting dimples (ductile fracture), after the nucleation and subcritical propagation of fatigue fracture.
36
VIEW
ML00081 / Aug.2017
Hydrogen Cracking of a Chromium Plated Carburized Pin
M - Low Alloy for Thermochemical TreatmentP - Chromium PlatedAp - AutomotiveCd - Hydrogen Embrittlement
Cm - Fractography by SEMDf - Dimples