Can steel stock be non-homogeneous ??

[Dan_the_Chemist]

I did a lot of woodworking at home before I finally bought the machines to be able to do metalworking at home. That taught me to be sensitive to the inhomogeneities in the wood stock I had.

Recently I have been working on some annealed 4140 drops from the ends of a 4.5" round bar. Over the last few days I have bored the center out, flattened one side to give a 2" wide flat, drilled 18 tap holes and tapped it with 7/16-14 threads to 1.5" deep.

While drilling the tap holes I noted that the behavior of the drill was not the same all the time. While drilling some of the holes the sound made by the drilling operation would sometimes suddenly change. This was often accompanied by a change in the nature of the chip. After 1/2" the sound would generally return to the original sound. The sound, resistance to hand feed, and the nature of the chip seemed to indicate a small region of different hardness.

Is that a reasonable explanation? Does raw steel stock occasionally have inhomogeneities like that? Or am I just missing something that I'm doing that changes the progress of the drilling operation slightly?

[JRouche]

I did a lot of woodworking at home before I finally bought the machines to be able to do metalworking at home. That taught me to be sensitive to the inhomogeneities in the wood stock I had.

Recently I have been working on some annealed 4140 drops from the ends of a 4.5" round bar.

Is that a reasonable explanation? Does raw steel stock occasionally have inhomogeneities like that? Or am I just missing something that I'm doing that changes the progress of the drilling operation slightly?

You say "raw" steel stock. What do you mean by that? Right from the foundry? Also, annealed drops. Who annealed the stock, maybe they didnt do a thorough enough job so there are inclusions of harder material. JR

[JoeLee]

You could be work hardening the material under the drill point. I've had the same issues drilling hot rolled products.

JL..............

[Dan_the_Chemist]

You say "raw" steel stock. What do you mean by that? Right from the foundry? Also, annealed drops. Who annealed the stock, maybe they didnt do a thorough enough job so there are inclusions of harder material. JR

The material was originally purchased from Alro Steel by a big machine shop in <city>. It was sold as "annealed 4140". I don't know who annealed it. The machine shop didn't. They bought it as "annealed". The machine shop bought it as long 4.5" rounds and cut the rounds to length. When the drops are pretty short they go into a scrap metal bin. I used to work with one of the guys, and he lets me dig in the bin as long as the boss ain't looking and I don't get greedy. Occasionally adult beverages change hands.

[rkepler]

I've had ends of 4140 round bar significantly harder than 4-5" into the bar.

[JoeLee]

I've had ends of 4140 round bar significantly harder than 4-5" into the bar.

That's interesting. I've seen that with bars that were sheared, hardened due to compression, but would they be shearing 4 1/2" rounds ?? Have noticed the same with the ends of stock where it was the end of the bar from the mill run. It probably cools faster there.

JL...............

[chipmaker4130]

The short answer, yes. It happens in any type of steel including stainless. Annealed 4140 is usually 'hot rolled' and can feel or sound different whenever there are slight variations in hardness. Since it takes you about 1/2" to get back to 'normal' I doubt inclusions are the issue, more likely its just random changes caused by uneven heating/cooling during processing.

[chipmaker4130]

I might add that you rarely find such variations in 4000 series HT or PH because the heat/cool cycle is more uniform.

[Doozer]

Anisotropic applies to steel for sure, as well as wood.
Very common to call out "grain" direction on prints
for deep draw stampings. It makes a difference if the
steel is going to tear or not.

--Doozer

[Rich Carlstedt]

I .... Does raw steel stock occasionally have inhomogeneities like that? .................

Absolutely !---in fact it happens all the time
There are "grades" to steel that most machinists are not aware of .
In our die shop, we only used AQ steel , which means Aircraft Quality ... totally different steel quality as extra steps such as ultrasound are taken to assure
homogeneous steel content and treatment.
Rich

https://www.steelwarehouse.com/aircraft-quality-grades/

[Bented]

You may find this surprising but the end and beginning of steel bar stock may have very different properties created during the manufacturing process.

[rickyb]

I think the crew is missing the mark here. This has to do with how the steel was stirred and how it was originally cast. Steel used to be cast in variations of 2’x3’ blocks.
It was later reheated and rolled down to the size required. In your case 4.5” provides a reduction ratio of 170:1. This means the area has been reduced that much which reduces the size of the original casting bubbles and inclusions by that much and also stretching them out hundreds of feet. This material would be very homogeneous regardless of heat treat.

Today there are only a few mills world wide that make steel this way. 99% is continuous cast in sizes 6-18” square or round. I’ve been in mills that spun the mold and others that used electromagnetic stirring. The spun billets move all the bubbles and inclusions to the center. The stirred billets are essentially as cast through out. This vestige of the original casting is most likely what you are experiencing.
A 4.5” round made from a 6” billet is essentially still a casting with only a 2:1 reduction ratio. Without knowing the mill there is no telling what you have. I was around for the industry changeover of the mills and found that getting below 6:1 reduction ratio was an issue in structural components.

[boslab]

Rolled steel, it’s made from cast slab, bloom, billet, ( and other varieties) during casting the chemistry of the outside of the cast slab is never ( without exeption)the same as the core, it’s the way dendritic growth occurs from outside of the cast slab ( or whatever) to the inside, the grain growth drives higher melting point stuff ( basic analogy) towards the centre or core, sampling the centre will give a different chemistry to the skin.
once cast the rolling begins, the average slab is about 9/10” ( the slabs I used to make were 234 or 254 depending on caster, but high speed plate casters seem to aim 100 or 80 mm ( that’s the mould thickness, width is generally adjustable in what’s called a remote adjustable mould and top zone)
6-10 passes through a reversing roughing mill to bring to thickness, big mills can take 254 down to 19 in 5 bites or passes, during this the grain structure is elongated obviously but importantly the centre is still different from the outside, there’s a bit of diffusion of carbon above 700c but analysis is still not homogeneous, also a defect like a spot of slag or mould powder can now stretch out to a km in length, without going much further into the process ( cold rolling, temper rolling etc,) centre and outside are different, not by a vast amount but more than enough to produce the effects your getting,
pone of the last ones I made was 350 tons of 4140 at 10” thick ( 254 act) and 1875 wide, I still have a bit of that heat, .
if your interested to see the insides you can do a sulphur print of the edge of a thick plate, quite revealing actually, it’s not hard if you want to know I can tell you how with some nitric, sulphuric and some old exposed black and white photographers paper.
mark

[Doozer]

Loving the information here.
Brain food.

-D

[RSG]

Loving the information here.
Brain food.

-D

Agreed!