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ML00148 - Sep.2018

Brittle Fracture of a 420 SS Barrel Nut

Microstructural aspect of a longitudinal section of the nut, in the middle of the thickness. Martensitic structure, banded.

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Microstructural aspect of a of the , in the middle of the thickness. Martensitic , banded.
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Material

Martensitic Stainless Steel
  • ASTM A 276 type 420
  • UNS S42000
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Description

When a hose clamp was disassembled it was detected that one barrel was broken, and the presented a brittle aspect. The material is a , type 420, and presented a high content (0.033%), although meeting the specified grade, suggesting that a temper embrittlement condition, during , could be responsible for the . The low hardness level and the morphology confirm the wrong tempering temperature.

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Chemistry (% in weight)

Nut analysis

0.282C; 0.34Si; 0.29Mn; 0.033P; 0.001S; 13.1Cr; 0.27Ni; 0.07Mo; 0.025Nb; 0.07Cu; 0.01Al; 0.039V.

Associated Images
Aspect of the barrel nut that is broken in a brittle way (without plastic deformation). The Nut diameter is 1", the nut length is 2" and the bolt thread is 1/2" (nominal diameter).
753
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Aspect Before AnalysisDf - General View
Visual aspect of both sides of the fractures. They present a macroscopically brittle aspect, with chevron marks indicating the starting point at the thinner edge of the thread.
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VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Aspect Before AnalysisDf - Fracture
The fracture is flat, in the region of the lower resistant section, presenting no macroscopic evidences of plastic deformation. The starting point of the fracture is indicated by the chevron marks.
62
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fracture Surface AspectDf - Chevron Marks
Low magnification micrography of the nut in the longitudinal section, presenting the machined internal thread and a banded microstructure.
55
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Banded Structure
Microstructural aspect of a longitudinal section of the nut, in the middle of the thickness. Martensitic structure, banded.
46
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Banded Structure
Microstructural detail of the banded structure. The rough aspect of the band is created by the carbides (better seen in the higher magnification image) in the segregation band.
38
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Banded Structure
Micrographic detail of the nut longitudinal section presenting the large carbides (white islands) in the segregation band, and small precipitates all through the martensite matrix, indicating that this high carbon steel was tempered in a high temperature. The 36HRc hardness also confirms this high tempering temperature.
39
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Martensite
Vickers microhardness identations using 100 grams. In a position out of the band (top line). Values from left to right, 380HV; 390HV and 394HV. In a position inside the band (bottom line). Values from left to right, 433HV, 483HV and 473 HV.
33
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Banded Structure
Macroscopically brittle fracture of the barrel nut, starting at the edge of the thread, as shown by the chevron marks.
29
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fracture Surface AspectDf - Chevron Marks
Aspect by scanning electron microscopy of the brittle fracture surface. in a low magnification view. The fracture presents smashed regions (conditions imposed after the fracture was opened) in the region where the chevron marks indicate it is the failure starting point.
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VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fractography by SEMDf - Brittle Fracture
Observing in higher magnification it is possible to see that the fracture presents a mixture of ductile and brittle micromechanisms. The intergranular mechanism indicates that the material is embrittled and the dimples indicate localized overloading conditions.
32
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fractography by SEMDf - Fracture
Detail of the ductile dimples and the brittle intergranular micromechanisms. This evidence (a mixture of mechanisms) suggests that the embrittlement is localized in some regions of the material.
34
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fractography by SEMDf - Fracture
Scanning electron microscope fractographic detail of the brittle intergranular micromechanism.
40
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fractography by SEMDf - Intergranular Fracture
Scanning electron microscope fractographic detail of the ductile dimples.
26
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Fractography by SEMDf - Dimples
Low magnification micrography of the nut in the longitudinal section, presenting the machined internal thread and a banded microstructure. It is possible to see that the contact pressure in the thread caused a surface plastically deformation.
24
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Banded Structure
Micrography of the nut in the longitudinal section, plastic deformation at the thread surface due to high local stresses, probably during assembling.
24
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Plastic Deformation
Micrographic detail of the nut in the longitudinal section, presenting cracks associated to the plastic deformation at the thread surface. These cracks were formed due to high local contact stresses.
19
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Plastic Deformation
Micrographic detail of the nut in the longitudinal section, presenting cracks associated to the plastic deformation at the thread surface. These cracks were formed due to high local contact stresses.
17
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Cracks
Micrography of the nut in the longitudinal section, transverse to the fracture (top of the image).
15
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Fracture
Micrography transverse to the fracture (top of the image) showing no relationship with the bands.
15
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Fracture
Micrographic detail of the fracture micromechanism showing no relationship with the bands and the intergranular morphology.
23
VIEW
ML00148 / Sep.2018
Brittle Fracture of a 420 SS Barrel Nut
M - Martensitic Stainless SteelP - Barrel NutAp - LiftingCd - Heat Treatment Defect
Cm - Kalling Etched Micrography by OMDf - Intergranular Fracture