Guide
Pre-hardened 4140 or annealed 4140 hardened after machining: which route fits your part?
Pre-hardened 4140 is chromium-molybdenum alloy steel bar supplied already quenched and tempered, typically to about 28–32 HRC, so it can be machined to finished size with no hardening afterward. Choose it when that hardness meets the requirement; choose annealed bar, rough machining, hardening and finish grinding when the drawing needs more hardness than pre-hard bar offers, and accept the extra steps and distortion allowance that come with it.
By Saola Manufacturing engineering team · Published · Updated
The grade choice between 4130 and 4140 is covered in the 4130 vs 4140 guide, and how much stock to leave for distortion is in the heat treat distortion guide. This page is the route decision for a 4140 part: buy the hardness in the bar, or put it in after machining.
On this page
What hardness does pre-hardened 4140 come at?
Distributor data typically give a nominal 28–32 HRC for 4140 HT or 4140 PH bar. Hardness falls off in larger sections, so the actual value depends on bar size and should be stated on the drawing.
One specialty metals distributor describes 4140 HT as a medium carbon alloy steel heat treated to a nominal hardness of 28–32 HRC, used for fixtures, clamps, base plates, tool holders and short-run dies (SB Specialty Metals, 4140 HT technical data). A steel maker's datasheet for quenched and tempered 4140 gives 269–331 HB for 10–100 mm sections and a lower 248–302 HB for 101–250 mm sections (voestalpine High Performance Metals, AISI 4140 datasheet), which is the section-size effect in numbers.
In the US the product is usually sold to ASTM A434, which covers hot-wrought and cold-finished quenched and tempered alloy steel bars in strength classes BB, BC and BD for hot-wrought bar and BB and BC for cold-finished bar (ASTM A434/A434M-24 scope). Many distributors sell it simply as 4140 HT, 4140 PH or 4140 Q&T with a hardness range on the certificate, so the drawing should say what range is acceptable rather than rely on the trade name.
When does pre-hardened 4140 make sense?
Use it when the part needs moderate strength and toughness, a hardness in the pre-hard range is enough, and you want the finished dimensions to come straight off the machine. Parts whose bores, threads and keyways must stay where they were cut are the typical cases.
The advantage is that nothing happens to the part after the last cut. There is no quench to distort it, no scale to remove, no decarburized layer to worry about and no separate furnace step to schedule. Tight bores, threads and keyways are cut once and stay where they were cut. The cost is that pre-hard bar machines more slowly than annealed bar and wears tools faster, which as typical guidance is a modest penalty with carbide tooling at this hardness.
Pre-hard bar can also take a surface treatment for wear. Both datasheets cited here state that 4140 is suitable for nitriding, and the distributor sheet lists ion, gas and salt bath nitriding for 4140 HT (SB Specialty Metals). Nitriding runs below the tempering temperature, so it adds a hard case without throwing away the core hardness; it is the usual answer when a pre-hard part needs a harder wearing surface.
When should you machine annealed 4140 and heat treat after?
When the drawing needs hardness clearly above the pre-hard range, or a through-hardness that pre-hard bar does not reach in your section size. The route is then rough machine, harden and temper, then finish the critical features.
The distributor's tempering response for 4140 shows why: oil-quenched 4140 tempered at 1000 °F gives about 36–37 HRC, at 800 °F about 45 HRC and at 600 °F about 50 HRC, against a nominal 28–32 HRC for pre-hard bar (SB Specialty Metals). Annealed 4140 is much softer, quoted on the same sheet at 185–200 BHN, and machines easily, but the part then moves in the quench.
That movement is why the route has three stages. Leave grinding or hard-turning stock on the features that matter, keep the others finished in the soft state, and plan the finishing operation after hardening. The decarburization guide explains why the surface layer must also be removed on load-bearing features.
| Requirement on the drawing | Route | Bar to order | Main risk |
|---|---|---|---|
| Hardness within about 28–32 HRC, or a strength level pre-hard bar meets | Machine pre-hard bar to finish | Quenched and tempered bar, hardness range on certificate | Section too large to meet the range at the core |
| Pre-hard core plus a hard wear surface | Machine pre-hard bar, then nitride | Quenched and tempered bar | Nitride case depth and growth on tight fits |
| Hardness clearly above the pre-hard range (for example mid-30s HRC and up) | Rough annealed bar, harden and temper, finish grind or hard turn | Annealed bar | Distortion, scale and decarburization |
| High hardness only on a journal or seat | Pre-hard or annealed bar plus local surface hardening | Depends on core requirement | Transition zone and distortion near the hardened area |
| Hardness not specified, only 4140 | Query before quoting | Undefined | Two suppliers quote two different parts |
How do section size and stress relief affect the choice?
Large sections of pre-hard bar are softer at the core than at the surface, and heavily machined pre-hard parts can move as stresses are released. Specify where hardness is measured and whether a stress relief is required between roughing and finishing.
The voestalpine figures show the section effect: the listed hardness range drops between the 10–100 mm and 101–250 mm size bands. If your part is cut from a large diameter and the core carries load, ask for hardness at a stated depth or location, not only on the bar surface.
For distortion during machining, the distributor sheet gives a stress relief for hardened material of 50–100 °F below the last tempering temperature (SB Specialty Metals), and the voestalpine sheet likewise says to stress relieve quenched and tempered material below the tempering temperature, so the relief does not lower the hardness. Plates and long, asymmetric parts that lose a lot of material benefit most; the stress relieving guide covers where it belongs in the sequence.
How should the drawing and RFQ call out pre-hardened 4140?
Name the grade, the condition, the hardness range and where it applies, and the certificate. For the heat-treat-after route, give the final hardness and mark which features are finished after hardening.
Generic callouts to adapt; fill the hardness range from your requirement, for example the typical 28–32 HRC of pre-hard bar:
MATERIAL: AISI 4140 STEEL BAR, QUENCHED AND TEMPERED (PRE-HARDENED), [RANGE] HRC. HARDNESS TO BE VERIFIED ON EACH LOT. MILL CERTIFICATE REQUIRED. NO FURTHER HEAT TREATMENT.
MATERIAL: AISI 4140 STEEL, ANNEALED. HEAT TREAT TO [ ] HRC AFTER ROUGH MACHINING. FEATURES [ ] FINISHED AFTER HEAT TREATMENT. REMOVE DECARBURIZED LAYER FROM [ ].
Write hardness as a range, not a single number, and say which scale applies. If the part will be nitrided, give the case depth and surface hardness requirement and state whether nitriding growth must be allowed for on fits. The Rockwell vs Brinell guide explains why a certificate in HB and a drawing in HRC need care, and the effective case depth guide covers case callouts.
How is a 4140 part handled on a Saola order?
4140 is not on the lead plant's standard turning bar list, so the grade, the bar condition and its source are confirmed at drawing review before quoting. Any hardening step is also confirmed at that review.
The standard bar materials on the CNC turning page are 52100, C1117 and 12L14 steel, 2011-T3 and 6061-T6 aluminum, silicon bronze and DOM steel tube. A 4140 drawing is reviewed for bar availability in the condition you call out, and the quote names where each step is done.
Inspection equipment at the lead plant includes a hardness tester, and material certification is available on request, as the quality page describes. Send the drawing for a quote with the condition and hardness range written out.
Frequently asked
Is 4140 PH the same as 4140 HT?
They are trade names for the same idea, 4140 bar supplied quenched and tempered to a pre-hard range. The names are not standards, so the drawing should state the hardness range.
Can pre-hardened 4140 be hardened further later?
Yes, it can be re-hardened like annealed 4140, but the part will distort and scale like any hardened part, so plan finishing stock. If you know you need more hardness, start from annealed bar.
Does pre-hard 4140 need stress relief?
Not always. Simple turned parts usually do not; plates and parts with heavy, one-sided material removal often benefit from a stress relief below the tempering temperature between roughing and finishing.
Can pre-hardened 4140 be welded?
With care. 4140 has high hardenability, so welding needs preheat and controlled cooling, and the heat-affected zone will not match the pre-hard condition. The 4130 vs 4140 guide covers why 4130 is the usual welded grade.
Is 4140 pre-hard hard enough for a sliding wear surface?
Often not on its own. A nitrided surface or a separately hardened wear part is the usual answer when a pre-hard part rubs under load.
Sources
- ASTM A434/A434M-24, Standard Specification for Steel Bars, Alloy, Hot-Wrought or Cold-Finished, Quenched and Tempered (scope), ASTM International, 2024
- 4140 HT Technical Data, SB Specialty Metals (metals distributor), 2022
- AISI 4140 / 1.7225 / G41400 datasheet, voestalpine High Performance Metals Australia, 2024 (file dated 2026 v1)