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Anonymous Poster #1

Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 6:23 AM

I am using EN8D unalloyed medium carbon steel material bright bars (dia 22 mm) for manufacturing a component which is used just like a gear.

Component Manufacturing process flow is turning, milling & drilling, gear teeth hobbing, induction hardening and OD grinding.

The problem which I am facing is, few component gets cracked during different stages of machining.

Earlier, i had doubted my hardening process but, for few components, cracks could be observed in soft form too. So that's not the root cause.

Crack is such minor that it's quite difficult to see by naked eye. In few components, cracks are not visible before hardening but cracks gets expand after hardening and grinding of components.

As material inspection method, I am doing following metallurgical testings before processing material: BHN Hardness test, Spectro chemical analysis for verification EN8D compliance, micro structure test to verify whether micro structure is normalised or not.

But problem exists even if material is OK as as per above mentioned testing methods.

Later on, I tried to test material in raw form with ultrasonic testing for detection of internal cracks. And found that material is satisfactory. But even after taking such care, components gets cracked during different stages of machining.

Can anyone please guide me regarding what might be the root cause for this issue. Also, please suggest me any further methods to identify defect at incoming stage of inspection.

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#1

Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 8:58 AM

This seems to me that the metal is being heated in one location causing stress cracks to develop...I would check the water flow cooling on the machine tools is working and adequate flow volume, and discourage overly aggressive machining...You might examine your process with IR imaging to look for problem area....or the cracks may be existing in the stock and are passing undetected until machining phase...

https://www.productionmachining.com/articles/stress-cracks-in-steel-bar-products(2)

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Anonymous Poster #1
#2
In reply to #1

Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 9:41 AM

Well, thanks for your valueable input. Your point is absolutely logical but as per my belief, cracks are not getting generated due to machining but it's just getting visible during machining. As you mentioned, if we consider it is inherent in stock material itself and is passing undetected during machining phase, then what's right method to detect it?

Actually, I turn the dia to 13 mm from bar of 22 mm for partial length of component and most of cracks are at this 13 mm dia portion. Cracks almost looks like a straight line.

Initially, I had tested stock material with dye penetration and magnetic particle testing for detecting surface cracks but as cracks are at the portion of dia 13 mm, I tried to test with ultrasonic testing too.

Is there any alternative methods to detect such defect?

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#3
In reply to #2

Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 10:12 AM

<...what's right method to detect it?...>

The Japanese method: reject the material and get on to the supplier to ensure that only 100% correct material is received! Only when the supplier is on-line with fully compliant material can this become a quality-assured production line.

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Anonymous Poster #1
#4
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Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 10:34 AM

LoL.That's what we have done as of now.��

But ultimately unavailability of raw material has affected production and main thing is how can I ensure the same thing whenever new lot is received. ����

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Guru
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#6
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Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/12/2021 2:32 AM

<unsubscribes>

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Guru

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#5
In reply to #2

Re: Identification Method for Defects in Raw Material of EN8D Bright Bar

01/11/2021 11:44 AM

Well defects exist in all manufactured materials, and that is what ends up in the scrap heap....Without control of quality control from the beginning of the process of steel making, you really have no control, so the best you can do is identify the bad parts, but what is an acceptable component failure rate for your company?...

..."The common ways in which quench cracking can occur from heat treating include:[4]

  • Improper steel selection: Selecting a steel with too high a hardenability can result in susceptibility to quench cracks and excessive core hardness.
  • Improper part design: Sharp changes of section, lack of radii, holes, sharp keyways and unbalanced sectional mass create stress risers in locations where cracking is likely to occur.
  • Inadequate stock removal: During original machining, remnants of seams or other surface imperfections act as a nucleation site for quench cracking during subsequent thermal operations.
  • Overheating: Temperature overshoot during the austenitizing portion of the heat-treatment cycle can coarsen normally fine-grained steels. Coarse-grained steels increase hardening depth but are more prone to quench cracking than fine-grain steels. Overheating and excessively long dwell times should be avoided while austenitizing.
  • Improper quenchant selection: Using an overly aggressive quenchant (e.g., water, brine or caustic) when a less severe quench media (e.g., polymer, oil) will work is a common cause of part cracking.
  • Improper fixturing and entry of the part into the quenchant: A part should enter the quench medium with as little interference as possible. Differences in cooling rates can be created, for example, if parts are nested together in a basket, resulting in the parts along the edges cooling faster than those in the mass in the center. Part geometry can also interfere with quenchant delivery and effectiveness.
  • Long lag times between quenching and tempering: Tempering parts as soon as practical will help avoid internal stresses from building up and being relieved by cracking. Certain high-hardenability materials, such as 4340, are particularly prone to quench cracking and must be tempered immediately (usually within 15 minutes of quenching) to avoid problems."...

https://www.industrialheating.com/articles/90314-quench-cracking

A forensic examination of the cracks is in order to determine what the source of the main problem is, is it just poor quality raw materials? ..improper handling?...Once that is determined, then the process of minimizing losses can begin...

The steel needs to be tested at each step of the manufacturing process to determine if the cracks are in the incoming stock, or are being produced by some step of the finishing process...

https://www.qualitymag.com/articles/92928-ultrasonic-testing-of-metal-bars-and-tubular-products

..."Micro-examination can help determine crack origin by noting orientation, intergranular nature, presence of scale, presence of subscale, and additional microstructural characteristics can reveal the thermal history of heating and cooling at the crack location."...

https://www.productionmachining.com/blog/post/stress-cracks-in-steel-bar-products

Two possible causes: 1) centerline segregation of impurities (occurs during solidification); or 2) central bursts (occur during diameter reduction).

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