(1) The document analyzes a steel bar that cracked during a bending test. (2) Microstructural examination revealed a martensitic surface zone with higher hardness than the ductile ferrite-pearlite core. (3) Chemical analysis found the steel to meet specifications. (4) The steel bar was likely overquenched during manufacturing, leading to a martensite area exceeding 30% and reduced ductility, causing cracks during bending.

1. BANGLADESH UNIVERSITY OF ENGINEERING AND TECHNOLOGY
MME 442
Bending failure analysis of a Steel bar
Submitted by:
Md. Mohaimenul Islam
ID: 1411004
Partners ID: i) 1411002
ii) 1411003

2. Visual Examination
Cracks larger than 3 mm
• During bending test, visible cracks larger than 3 mm were found.
• Produced by AKS
• TMT 500 grade Steel bar
• Wasn’t completely failed but surface was cracked.
• Ductility was reduced.
• Overquenching, large martensite area could be the possible reason.

3. 500 GRADE TMT BAR
Fe500 is a grade of High Strength Deformed Steel Bar for Concrete Reinforcement. The number
500 usually denotes the minimum Yield Strength (in N/mm2) of the Bar to qualify as Fe500.
TMT refers to the thermo-mechanical treatment. A metallurgical process that combines
mechanical deformation (like compression or forging, rolling etc.) with thermal processes (like
heat-treatment, quenching etc).

4. Microstructure zone of a TMT bar
Transition zone
(Bainite)
Tempered Martensite
zone
Ferrite pearlite zone

5. Chemical Analysis
Elements Fe C Si Mn P
Percentage(%) 98.667 0.20362 0.14429 0.86150 0.00495
S Cu Cr Al N B
0.00662 0.08330R 0.02480 0.00008R 0.00276R 0.00094
• First we cut a small piece from the specimen using saw.
• Grinded both cross sectional side of the specimen.
• Tested optical emission spectroscopy(OES) for chemical elements.

6. Hardness
77.5
80.6
81.8
96.3
98.7
Before taking hardness, we have to grind the both cross sectional side of the specimen. Then
we measeured the hardness using the Rockwell Hardness Tester.
Hardness of the core is lower than the case.

7. Microstructure
To observe microstructure, we had to take the specimen and grind both side of the
specimen for sectioning.
Then we cleaned the sample and hand grind it with emery papers of
80,120,320,600,800,1200,1500-grits for a smooth surface.
Polishing is the last step in producing a flat, scratch-free surface. After grinding
with 1500 grade emery paper, we polished it in a rotating wheel for 20 minutes.
Then we washed the sample and immediately after washing, etched the surface with
5% Nital.
After etching, we took the sample under optical microscope and observed the
microstructure of the sample.

8. Our sample after etching

9. Sample with 200x zoom

10. CORE MICROSTRUCTURE

11. TRANSITION MICROSTRUCTURE

12. CASE MICROSTRUCTURE

13. Tempered Martensite at the
surface (500X)
Bainite at the transition
zone (500X)
Ferrite pearlite at the core
(500X)

14. Our sample Perfect sample
Larger Martensitic
Area

15. several diameters of each sample using scale were taken and
then averaged .Then several diameters of ferrite-pearlite zone
was measured from each sample and averaged.
Area of Martensite region: Am =
1
4
𝜋 14.372−10.292
1
4
𝜋(14.372)
∗ 100
= 48.75%
Area of Martensite region: Am =
1
4
𝜋 16.242−13.622
1
4
𝜋(16.242)
∗ 100
= 29.66%
Area of Martensite should be about 30%

16. Discussion
• From the visual observations, we observed the cracks of the bar.
• From the chemical analysis, we found the chemical components of the sample. Here everything
was fine and no suspicious elements were found.
• From the hardness analysis, we found higher hardness at the case than in the core. It indicates it
was TMT bar.
• From the microstructure analysis, we found different structure at the core, transition and case
zone.
• After etching, the area of every zone was visible. The area of martensite, Am was measured and
compared to another perfect sample.
• The total martensitic area was higher in our sample.
• The rebar got over quenched while it was passing through the waterbox during hot rolling.
Thus, martensite area was increased than its’ critical value and the surface got cracked during
bending test.

17. CONCLUSION
• Rebars were over quenched in waterbox due to mismatch in water flow through water
box. As a result, martensitic area grew larger than its’ critical value. So, ductility was
reduced and hardness profile increased.
• During bending test, it couldn’t take the bending load and crack was formed at the
surface.
• But it didn’t fail catastrophically as in the the core there were ferrite-pearlite region
which were soft and ductile.

18. THANK
YOU