Thread Rolling Before or After Heat Treatment?
Two purchase orders can call for the same bolt and thread class, but put very different blanks in the rolling machine. In one plant the blank goes to rolling before its final strengthening heat treatment. In the other it arrives at the rolls already heat treated. Give the die maker that sequence before asking for a price. A tooling trial on the first blank does not qualify the second.
Start with the route on the part drawing
Check whether the drawing, purchase specification or approved process already fixes the sequence. If it does, quote to that route and treat a proposed change as an engineering change. If it leaves the choice open, agree who will approve the route and what evidence is required. A buyer can specify the rolling machine and a die maker can recommend tooling, but approval of fatigue performance belongs with the people responsible for the fastener and joint.
For rolling before heat treatment, send the condition of the blank at the rolls and show the heat treatment and finish that follow. A trial can show that the die forms the required profile in that incoming condition. Plan to inspect the part again at the stage specified for acceptance. If the as-rolled thread passes and the finished part does not, keep a traceable sample or measurement from each stage. The point where the result changes is useful evidence before anyone orders a different die.
For rolling after heat treatment, send the actual hardness range at rolling, its scale and where it was measured, along with the steel grade, surface condition and measured blank diameter. The final property class alone does not describe what the dies will see. Provide the machine model, roll or flat-die arrangement and available setup envelope. Have the toolmaker review that combination before assuming the old die and machine settings will carry over.
What the fatigue tests actually compared
Rolling can leave compressive residual stress near the thread root. A later heat treatment can change that stress state. This is one reason the sequence matters on some fatigue-sensitive bolts. Its effect on a loaded joint depends on the material, manufacturing route and test loading. In a 2007 JSME experiment, the benefit reported for rolling after heat treatment was larger at low mean stress and narrowed as mean stress increased. A design team should compare its own load case before carrying any published benefit into a specification.
A separate 2007 ASTM symposium study on coarse-thread bolts makes the test boundary concrete. The authors used property-class 12.9 bolts with a 3/8-16 coarse UNRC thread. They normalized preload to each route's measured proof stress. Both routes were tested at matched absolute preloads. The percentages differ because the reported proof stress for the specimens rolled after heat treatment was 10% lower in that study.
| Test condition | After versus before heat treatment |
|---|---|
| Lowest preload, nominally 1% of the proof stress of bolts rolled before heat treatment | The authors reported 147% higher fatigue strength at 10 million cycles for the threads rolled after heat treatment. |
| Higher preload conditions in the same study | The authors reported no increase in fatigue strength from rolling after heat treatment. |
Here, the reference proof stress is that of bolts rolled before heat treatment. The five test preloads were 1%, 50%, 75%, 90% and 100% of that reference value. Those are test inputs. The 147% result describes fatigue strength at the stated cycle count, not an increase in the number of cycles. Neither study supplies a die life, a permissible rolling hardness or an acceptance limit for another bolt.
If buying for a fatigue-sensitive joint, give the design or quality team the actual fastener specification and proposed route. They can define the joint configuration, load range and acceptance criteria for any comparison. Ask the tooling supplier to quote against the agreed blank condition and first-piece requirements. A die quote and a fatigue test answer different questions, and both need to refer to the same route.
When the trial points to a process problem
Consider a shop moving rolling to after heat treatment on a steel bolt. The old die set gave acceptable threads on the original route, but the new first-piece report shows an incompletely formed crest and the machine load moves away from the accepted baseline. Before ordering a revised profile, compare the blank diameter and surface with the trial plan and confirm that its hardness was measured on the material that actually went through the rolls. Record the existing die condition, the machine settings and the positions where the form departs from the drawing. Retain a pre-roll blank, a good piece from the old route and the affected first piece.
If the blank lot falls outside the agreed incoming condition, investigate that difference before using the trial to judge the die. If the incoming condition is confirmed and the defect repeats with traceable settings, send the three samples and the inspection record to the supplier. Ask whether to review the entry and working profile, the existing tool condition or the machine setup first. Keep the parts quarantined under the plant's normal nonconformance process when acceptance has failed. A single rise in load or a weak crest does not identify the cause.
When an as-rolled pass becomes a final inspection failure
On the roll-first route, a different pattern is possible: the same lot passes thread inspection as rolled and fails after the subsequent treatment or finish. Check that the before and after records refer to the same drawing and to the intended inspection conditions. Record whether the change is limited to one measurement or appears across the profile, and verify the gauge and its setup. Ask the process owner to review the intervening operations with those records in hand. A failed final gauge result by itself does not tell the die maker which feature to alter.
Put the acceptance stages on the purchase order
- Before rolling: Identify the blank material, lot, measured diameter and actual hardness condition. Attach the operation sequence, including the location of the final strengthening heat treatment and any subsequent finish.
- After rolling: Tie the die set and machine settings to the first-piece report. Record the specified gauges, actual results and any separately required thread measurements. Keep the trial sample identified by lot.
- At final release: Agree which checks repeat after subsequent treatment or finishing and who owns any proof or fatigue test. A ring gauge result provides no fatigue data.
For tooling, see the flat dies for screws and bolts and the thread rolling die range. The blank diameter guide covers the pre-roll dimension in more detail. Send the bolt drawing, process route, blank hardness and first-piece results to Jungu for a die review.