CNC tensile strength of metal parts: what sets it, what erodes it

• 1
  Five things to hold on to
• 2
  What tensile strength actually measures
• 3
  How alloy and temper set the ceiling
• 4
  Where CNC machining helps and where it hurts
• 5
  Grain direction and section design
• 6
  How to confirm the strength you specified
• 7
  Alloy and temper: typical tensile ranges

Tensile strength is the peak stress a material carries while being pulled before it necks and fractures. Test it on a standard coupon and you get three useful numbers: yield strength, where permanent deformation starts; ultimate tensile strength, the peak; and elongation, how far it stretched. For a bracket or a shaft, yield usually governs the design. Ultimate strength tells you the safety margin beyond yield.

Units are MPa or ksi. Aluminum 6061-T6 sits near 310 MPa ultimate and 276 MPa yield. 7075-T6 reaches roughly 572 MPa ultimate. 316L stainless lands around 485 MPa. Titanium Ti-6Al-4V annealed is about 950 MPa. These are textbook ranges for standard tempers, not a promise about the bar you bought.

The number only applies to the condition tested. A supplier who quotes 6061 without a temper is quoting half an answer. T6 and O differ by a factor of three or more. When we review a drawing, the first thing we check is whether alloy and temper are both on it.

Tensile strength also changes with temperature and strain rate. Aluminum loses strength steadily above 150 °C. Most steels hold until several hundred degrees. A part that passes a room-temperature pull test may still yield in a hot engine bay.

• 1
  Yield vs ultimateYield sets the working limit. Ultimate sets the failure ceiling.
• 2
  Temper is part of the specT6, T651, H32, annealed. Each one is a different material in practice.
• 3
  Direction mattersRolled and extruded stock is stronger along the grain than across it.
• 4
  Temperature derates itAluminum especially. Check the service temperature before you size the section.

Alloy choice sets the band you can work in. If a part needs 500 MPa, no amount of careful machining will get 6061-T6 there. You move to 7075, 17-4PH, 4340 or Ti-6Al-4V. That decision belongs in design review, not on the shop floor.

Temper does the rest. 17-4PH is a good example. In the annealed condition it is soft and machines easily. After H900 aging it reaches about 1,310 MPa ultimate. But H900 also makes it harder to cut and more prone to distortion during finishing. We often rough in the annealed state, age the part, then take a light finishing pass. That sequence costs one extra operation and saves a scrapped batch.

Aluminum tempers work the same way. 6061-T6 is the default for structural brackets because it is weldable, corrosion resistant and available in plate and bar. 7075-T6 is stronger but less weldable and more sensitive to stress corrosion. 2024-T351 machines well and is common in aerospace, but it needs protection against corrosion.

Stainless is a separate conversation. Austenitic grades like 304 and 316L cannot be hardened by heat treatment. They work-harden instead, which means light finishing passes can leave a harder, stronger surface than the core. Martensitic grades like 440C and 420 can be hardened and tempered, but they move more during heat treatment.

• 1
  Pick alloy firstIt sets the achievable range. Machining cannot exceed it.
• 2
  Then pick temperT6 versus O is a three-fold difference in the same alloy.
• 3
  Sequence precipitation gradesRough soft, age, then finish. It limits distortion.
• 4
  Know your stainlessAustenitic grades harden by working. Martensitic grades harden by heat.

Machining is subtractive, so it does not add strength to the material. What it can do is preserve the strength that is already there, or quietly remove it. The difference usually comes down to heat, tool pressure and residual stress.

Cutting temperature is the biggest lever. If the edge runs hot enough to exceed the tempering range of the alloy, the surface layer softens. On hardened steels that shows up as a soft skin a few hundredths of a millimeter deep. On aluminum it shows up as built-up edge and a torn finish. Both are avoidable with the right speed, feed and coolant.

Residual stress is subtler. Rolled plate and extruded bar carry internal stresses from the mill. When you remove material from one side, the balance shifts and the part bows. A long thin rib is the classic case. Rough it, let it relax, then take a finishing pass. Sometimes an intermediate stress-relief cycle is worth the extra day.

Thin walls behave the same way. Below about 1 mm on aluminum, chatter and deflection start to dominate the cut. The tool pushes the wall away, the chip thins, and the surface finish suffers. Reducing radial engagement and supporting the wall from behind usually fixes it.

• 1
  Watch cutting temperatureStay below the tempering range or you soften the skin.
• 2
  Rough, relax, finishThe standard fix for bowing on long parts.
• 3
  Thin walls deflectBelow roughly 1 mm in aluminum, support the wall or reduce engagement.
• 4
  Coolant is not optionalOn hardened and gummy alloys it controls both heat and chip evacuation.

Wrought stock is anisotropic. A rolled plate is stronger in the rolling direction than through its thickness. An extrusion is strong along its length and weaker across it. If your part sees its highest load in one direction, orient the blank so that direction aligns with the grain.

This is a design decision, not a machining one. Once the part is programmed, the orientation is fixed. Changing it later means re-fixturing, re-programming and often a new blank. Bring it up before the first cut.

Section design matters too. Sharp internal corners concentrate stress. A fillet radius of at least one third of the section thickness spreads that load. Threads cut directly into a thin wall will strip earlier than the parent material suggests. A boss or a threaded insert is usually the better call.

Surface finish is the last variable. A rough turned surface is a row of small notches. On a fatigue-loaded part, that is where cracks start. Going from Ra 3.2 μm to Ra 0.8 μm can make a measurable difference on a shaft, even though the bulk tensile strength is unchanged.

• 1
  Align load with grainFix orientation before programming, not after.
• 2
  Radius the cornersOne third of section thickness is a practical starting point.
• 3
  Do not thread thin wallsUse a boss or an insert instead.
• 4
  Finish affects fatigueBetter surface finish delays crack initiation.

Tensile testing is destructive, so you rarely test the part itself. You test a coupon cut from the same lot, or you test a sacrificial part from the same run. ASTM E8 and ISO 6892 define the specimen geometry and the pull rate. The result is only as good as the traceability behind it.

Hardness testing is the practical shop-floor proxy. For steel, the relationship between hardness and tensile strength is well established, so a few Rockwell or Brinell readings can flag a tempering mistake quickly. For aluminum, hardness correlates more loosely with temper, but it still catches a missed aging cycle.

For production parts, the usual package is material certification from the mill, in-process dimensional checks, and a final inspection report. If a customer needs mechanical properties verified, we cut a coupon from the same bar and send it out. That adds time but removes the argument.

Non-destructive methods exist. Ultrasonic and X-ray inspection find internal defects that would lower effective strength. They do not measure tensile strength directly. Use them to find flaws, not to certify a number.

• 1
  Test a coupon, not the partASTM E8 or ISO 6892 geometry keeps the number comparable.
• 2
  Hardness as a proxyFast, cheap, and good at catching tempering errors.
• 3
  Certifications and reportsMill cert plus in-process checks plus final inspection.
• 4
  NDT finds flawsUltrasonic and X-ray do not replace a pull test.