The bore measures good at the machine. Then it moves.
P550 and 15-15HS MAX are chosen for one reason — staying magnetically invisible around MWD/LWD sensors. They are difficult for another: they store elastic energy as they are cut, so the bore relaxes after the tool has passed. Here is the mechanism, and the four things to put in the RFQ because of it.
Most hard materials are hard in one way. These alloys are hard in two, and the two compound:
Spring-back. The alloy stores elastic energy as the cutting edge pushes through it. While the tool is in the bore, that energy is held. After the tool passes, stress redistributes, the material relaxes, and the bore drifts. A bore that measured in tolerance at the machine can end up out of tolerance once it is cool and free.
Work-hardening. Like their nickel-alloy cousins, these austenitic grades harden under the edge. Rub them with a dull tool and the surface hardens mid-cut, so the next pass is cutting something harder than the drawing specifies — and the pass after that harder still.
Neither mechanism is exotic. What makes these alloys a specialist job is that they arrive together, in the same cut, on a part whose tolerance is usually tight.
Spring-back and work-hardening both scale with how far the tool has to travel. In a shallow bore the material relaxes into a wall that is close by, and the geometry barely notices. In a deep bore there is nothing to hold the bore on line: every millimetre of relaxation adds to the next, and the deviation accumulates along the length.
This is why the same alloy that machines acceptably as a short part becomes a genuine problem at ten diameters, and a specialist job at a hundred. It is also why non-magnetic drill collars — long, deep-bored, tight-tolerance, and made from exactly these grades — are a useful test of whether a shop has actually done the work.
On a relative machining-difficulty scale — carbon steel = 1.00, higher means harder to cut — here is where these alloys sit:
Relative machining-difficulty coefficient — carbon steel = 1.00, higher means harder to machine.
P550 sits at 3.47 — roughly three and a half times the cutting resistance of carbon steel, and in the same league as titanium. Note the direction of the scale: this is a difficulty index, not a machinability rating. Higher means harder to cut.
Because these alloys move, the useful information is not only what the part is, but where the effort belongs:
This is the reason you chose the alloy, so state the number you need — and say who will measure it. Permeability is a specification, not a certificate: the shop should be verifying incoming material against your figure, not against a mill’s word
P550, 15-15HS MAX and the other high-nitrogen austenitic grades are not interchangeable, and the same grade behaves differently stress-relieved and as-rolled. If the blank is already stress-relieved, say so — it is the single most useful sentence you can add about material
A deep-bored collar has a handful of dimensions that decide whether the tool works downhole, and a long list that merely have to be reasonable. Marking the difference lets the route spend its effort where the function is — and stops a tight callout on a non-functional diameter from driving the price
Plunger and barrel pairs, seal faces, bearing seats: if two parts work together, they have to be made as a pair, because a bore that relaxes after machining relaxes differently depending on when it was cut. Say so on the drawing
None of these four is about how the part is made. All four change what the shop can promise.
Because the alloy is chosen for a magnetic property, the paperwork around it matters as much as the machining:
The mill origin follows the part through every operation to the inspection report
Optical emission spectrometry on every incoming heat, including nitrogen, the element that defines these grades
Tensile and impact testing on incoming material
Against the figure on your drawing — not against a generic threshold
Permeability is worth one more sentence, because it is the one item buyers most often treat as paperwork. MWD/LWD tools navigate by magnetometer and measure the formation by electromagnetic signal. A collar with a stray magnetic signature distorts the compass reading and the tool stops knowing where it is. That is a functional failure, not a documentation gap.
Yes — non-magnetic drill collars in these grades are in batch production here, with deep bores production-proven to 10,000 mm. What we will not do is promise a tolerance before seeing your print.
Because these austenitic alloys store elastic energy during the cut and release it afterwards. Stress redistributes, the bore drifts, and the effect compounds with length. It is a property of the material, not a machining error.
Yes, and it is the most useful single number you can give. It is the reason you chose the grade; without it, the shop can only verify against a generic threshold.
No — they are different grades that share the high-nitrogen austenitic family. They are not interchangeable on a drawing, and both are available here.
Attach your drawing and the material spec, including your permeability requirement. Within 24 hours an engineer — not a salesperson — replies with manufacturability feedback. NDA welcome.
