Hardness of Quench and Tempered alloy steels is a function of the tempering temperature. The higher the tempering temperature, the lower the hardness.
This is called an inverse relationship.
And it’s why some people call tempering “drawing.”
The temper “draws the hardness out of the steel.”

Normalized at 1600F, Quenched in oil 1550 F, Tempered 2 hours

 These curves are a rough approximation of the as tempered brinell hardness for the grades shown. For example, I have other data for 4340 that shows 440 BHN at 800F; 410 at 900F; 380 at 1000F; 340 at 1100F, and 310 at 1200F temper temperature.
Fire! Can't do a blog on heat treat without a picture of fire.

Your mileage may vary, in other words, but this graph is close enough for ‘considered judgement.’
Additional 4140 data that I have from my notes suggests 397 BHN at 800F; 367 at 900F; 335 at 1000F; 305 at 1100F and 256 at 1200F.
If you have better data from your process – USE IT.
Better yet, if you have time, send a sample to your heat treater for a pilot study.
In the absence of data from your process, the above figure and data will give you “a place to stand” in understanding what is possible when heat treating .40 carbon alloy steels- the steels most commonly encountered in our precision machining shops for Automotive, Aerospace, Agricultural and general applications.
Here is a video from PMPA member company Nevada Heat Treating to give you an inside look at what goes on at a heat treat service provider.

And how to minimize them.
Upon heating, steel parts change volume as they change crystal structure (I’ll spare you the technical terms and details). When these heated parts are quenched, their internal crystal structure changes again, and that volume change is not necessarily sufficient to offset the change upon heating. This change of volume can cause  dimensional distortion.  The rule of thumb that I have used for medium carbon alloy steels is  to expect a change in linear dimensions of about 0.125% maximum. That is, one eighth of a percent of the linear dimensions could be the change encountered  from heat treatment and quench. It generally is less, but 0.125% gives me a rule of thumb to evaluate capability to hold dimensions after heat treat. What rule of thumb do you use to estimate part growth as a result of heat treat?
Warpage or shape distortion  as a result of heat treat is different because it is usually a result of process and design issues rather than the expected phase changes of the material.

Salvador Dali understood!
Salvador Dali understood!

Here are 8 reasons steel parts can warp upon quench and tempering:

  1. Rapid heating.
  2. Overheating.
  3. Non-uniform heating.
  4. Non-uniform cooling.
  5. Non-uniform agitation.
  6. Water contamination in oil. 
  7. Large changes of mass and section.
  8. Asymmetric features.

Rapid heating can cause stresses to develop in parts due to excessive temperature gradients. Overheating similarly lowers mechanical properties,  potentially leading to parts sagging or creeping depending on orientation in the furnace. Non-uniform heating also creates differences in properties within the parts as well as leading to incomplete transformation products or hybrid structures upon quenching. Non-uniform cooling allows unbalanced stresses to develop during the quench, as does non-uniform agitation of quench medium.  Often non-uniform heating or cooling result from the way parts are stacked or piled in the basket or on the belt such that gradients of temperture are created. Water contamination in oil. This is difficult to figure out, but in addition to warped parts, inconsistent hardness readings between parts or on the same part are a sign of this. Parts with large section changes or that have asymmetric features are also more likely to warp than parts with balanced and uniformly distributed mass, regardless of process control.
Choosing steels with higher hardenability (alloys rather than plain carbon steels), finer grain size, and paying attention to the details of loading, time at temperature, and quenchant delivery are all steps that can minimize warpage distortion, even when part design is less than optimum.