Heat Treating Issues with 4140 (Scaling)

[sphelps]

Hi all, we've recently switched to 4140A from 4340A for some of the parts we manufacture. The 4140 is simply easier to come by and significantly cheaper in our location.

For the 4140 we soak the parts for 30min once the oven returns to it's set temperature of 1580F. The parts are 1" square.

They are then oil quenched (oil temp is 150F) and the hardness is 55RC.

Tempering is done at 1000F for about 30min to achieve a hardness of 45-46RC.

The procedure for 4340 is very similar except the tempering temperature is lower.

The problem is we're getting scaling on the 4140 which effects the surface finish quality, the scaling doesn't come off and actually looks more like lumps that form on the surface (perhaps scaling is the wrong word). If we reduce the load (less parts) the effect seems to be reduced.
This problem doesn't occur with 4340.

Any thoughts?

Here's a picture, the effect is most noticeable after plating.

 

[mighoser]

I'm assuming that if you plated unhardened parts these features wouldn't be present. How long between quenching and tempering? Your tempering temp is a bit low (1025F Min) and hold time is too short. Should be 1hr/inch section.

 

[swall]

The "alligator skin" like surface appearance (the round patches) suggests to me that the parts were overheated in the furnace during austenization.

 

[sphelps]

Thanks for the replies. Yes the features are only present after heat treating, the parts are machined from round bar.

Parts are usually quenched and tempered in one day, tempering is likely not an issue since the features are present after quenching.

I've tried a longer soak time and gotten the same results. Although the largest cross section is 1" the parts have cut out sections and features which can be as thin as 0.1", these are the critical sections for hardness.

The parts could have been overheated, this a possibility and could have occurred during the recovery of the oven temp. It would explain why the results get better with smaller loads. I will look into this. Thanks again.

 

[Robertmet]

To me that texture indicates decarb ! Check preheat treat sample for decarb and check you're atmosphere.
Temper should be at least 1 hour per inch.

 

[redpicker]

You say the scaling does not come off, but they are plated. You must be blasting the surface prior to plating, correct? That should remove all the scale. Any scale left on the surface would probably interfere with the palting. What kind of plating are you doing?

Are these being heat treated in an open atmosphere furnace, or a controlled atmosphere?

Are these stacked on each other while in the austenitizing furnace? It kind of looks to me as if the "spots" were areas where there was very little scale where the remainder is where there was a heavier scale. After cleaing, the spots end up being a bit raised.

The difference between 4340 and 4140 is mainly the nickel content. The higher nickel in 4340 will make the scale a bit more tighter and more difficult to remove. Perhaps this difference in the scale is making a difference in the shorter furnace times you are using.

rp

 

[sphelps]

The furnace is open atmosphere (uncontrolled), we can't temper the parts for more than 30min at 1000F as the resulting hardness will be too low, we could lower the temperature but I don't see the advantage. How would I check the pre-heat treated steel for "decarb"? The steel is 4140 Annealed Round bar, parts are machined prior to heat treating.

Yes it's more of a deformation than scaling, it just looks like scaling that doesn't come off. The parts are sandblasted after heat treating, prior to plating, the features are more visible after plating which is why I posted that picture.

 

[sphelps]

Parts are nickel plated

 

[unclesyd]

Are the heating elements elements in the furnace exposed?

Have you Austenitized a piece of old 4340 along with the new 4140?

How are the parts machined to size?

Your bar would have to severely mistreated while annealing to have a decarb layer. If you are machining over 0.065" off the OD you should see no decarb. Decarb can quickly be determined by a microscopic evaluation.

 

[redpicker]

Are these square bars stacked on top of each other during heat treatment?

Are all bars in a heat treat load like this, or only some?

Are all sides of the bars like this, or only the tops?

rp

 

[sphelps]

The furnace is gas powered, it's a brick oven, the heat is blown in through the base and distributed upwards and out the stack.
We only heat treat the same material in each batch so we don't mix the 4340 with the 4140.
The parts are machined to size prior to heat treating with CNC milling machines which use dry cutting inserts.

 

[sphelps]

The bars are not stacked, they placed in a tray with at least an inch of separation between other parts.

All bars within the load are the same.

All sides, but seems to be slightly concentrated on one end, the end near heat source which could result from overheating as previously mentioned.

 

[Metalguy]

Try lowering the aust. temp. to 1540-1550F. Also, don't you use ~1475 deg for 4340? Why 4340 for a 1" part? 150 deg oil is getting up there a bit.

"You see, wire telegraph is like a very long cat. You pull his tail in New York and his head is meowing in Los Angeles. Do you understand this? Radio operates the same way: You send signals here, they receive them there. The only difference is there is no cat." A. Einstein

 

[unclesyd]

Is it possible that you haave flame impingement on the parts?

 

[metengr]

Simple advice - have one of the affected pieces sent to a metallurgical lab for proper evaluation.