Non-Magnetic P550 Machining | Zhongnan Machinery

Why Non-Magnetic Alloys Are Hard to Machine

P550 and 15-15-class grades are high-nitrogen austenitic stainless alloys. They are chosen for one reason — staying magnetically invisible around MWD/LWD sensors — and they are feared for another: spring-back in deep bores.

These alloys store elastic energy as they are cut. The tool passes, the bore measures in tolerance at the machine — and then the material relaxes. Stress redistributes, the bore drifts, and a 10-meter hole that was good warm can be out of tolerance by the time it cools. Controlling it is not one clever setting — it is stress-managed sequencing, symmetric material removal, intermediate stabilization, and a route written by people who have watched a bore move and learned why.

On top of that, these austenitic grades work-harden like their nickel-alloy cousins: rub them with a dull tool and the surface hardens mid-cut. Spring-back plus work-hardening, in a 10-meter hole — that is the exam question. We pass it in production, not in a brochure.

Large bored part with a deep-hole tool approaching the bore — The exam question — a 10 m bore that must stay straight

The Difficulty Gap, in Numbers

Spring-back and work-hardening are not a P550 quirk — they scale with the material. On a relative machining-difficulty scale (carbon steel = 1.00, higher means harder), here is where P550 and 15-15-class grades sit against the alloys we run every day:

Nickel superalloysInconel 718, Incoloy 925

9.26

Titanium alloysTC4 / TC11 / TC18

4.17

High-nitrogen austenitic stainlessP550, 15-15 class

3.47

Martensitic stainless

1.98

Carbon steelbaseline

1.00

Relative machining-difficulty coefficient — carbon steel = 1.00, higher means harder to machine.

P550 sits at 3.47 — about three and a half times the cutting resistance of carbon steel, and in the same league as titanium. That number is the reason deep bores spring back: the harder the material pushes back, the more the process has to manage stress instead of simply cutting it away.

Capability — P550 / 15-15 Class × Our Processes

Process	Envelope / range	Notes
Deep-hole drilling (BTA)	Ø20–360 mm; machine envelope 12,000 mm; production-proven Ø20–300 × 10,000 mm	Straightness 0.5 mm/m; deviation within 0.8 mm over a 3,000 mm single-pass run
Gun drilling	Ø2–40 mm, depths to 6,000 mm; ±0.02 mm, Ra 1.6 μm	13 machines, all designed and built in-house
Deep-hole honing	Bores Ø100–400 mm, lengths to 6,000 mm (four FWHM-4000, self-built)	Corrects size and roundness over full length
CNC turning	To Ø800 × 8,000 mm (FWT-80800); shop envelope Ø1,000 × 10,000 mm	Slender shafts to 4,500 mm handled daily — steady rests, sequence planning, stress management
Precision grinding	Cylindrical to φ500 × 3,000 mm (MQ1350B), φ125 × 500 mm (MGB1412); internal φ200 × 250 mm (×2, Swedish)	To 0.005 mm, finishes to Ra 0.4 μm
Welding	Narrow-gap butt welding of non-magnetic steels (groove angle within 2°, depth to 60 mm); TIG and laser welding of P550 / 15-15-class alloys	In-house repair and fabrication
Threading & gauges	API thread gauge suite: REG / NC / FH profiles — 24 gauges + 64 setting plugs	API Spec Q1 / 7-1 / 7-2 licenses held by our JV Zhonghai Wantai
Verification	Non-magnetic calibration range (all-wooden, magnetometer-monitored) via JV; PMI by optical emission spectrometer	Permeability verified, chemistry verified

Where We Run It

Non-magnetic drill collars in batch production. P550 collars — including instrumented variants for MWD spacing — run in batch quantities on our Drill Collars line; 15-15-class non-magnetic drilling components run alongside them. To our knowledge we are one of the few shops in China with this record — and we say “few” because we can only speak for what we have verified.

Why permeability control matters. MWD/LWD tools navigate by magnetometer and measure the formation by electromagnetic signal. A collar with stray magnetic signature distorts the compass reading — the tool stops knowing where it is. That is why permeability is not a paperwork item here: incoming material is verified by PMI, and magnetic performance is checked on a non-magnetic calibration range — all-wooden, magnetometer-monitored — available through our JV.

Proven in the service envelope. The offshore downhole tools we supply components for operate at 180 °C / 180 MPa, proven in sustained field operation. Our JV Zhonghai Wantai holds API Spec Q1, API 7-1 and API 7-2 licenses.

Drill collars and long parts staged in rows — Drill collars — batch quantities

Optical emission spectrometer — PMI on every incoming heat — PMI at the door — nitrogen included

Machining Strategy

Speeds and feeds are earned per part and frozen into the routing. The doctrine we can share:

Symmetric, sequenced material removal
Bore and OD allowances balanced so residual stress relaxes toward straight, not toward scrap
Intermediate stabilization
The route pauses to let the material settle before the finishing passes, because a bore that moves after the tool is a scheduling problem as much as a cutting problem
Sharp tooling, firm engagement
No dwelling, no rubbing; in austenitic grades a worn edge manufactures a hardened skin
Measurement at temperature, and after
Critical bores checked cold, because “in tolerance at the machine” is only half the sentence
Threads and connections gauged, not assumed
REG / NC / FH profiles verified against calibrated setting plugs

Gun drilling a long part — coolant at the bore entry — Deep-hole work — where stress management decides the bore

Traceability & Verification

The alloy is chosen for a magnetic property — so the paperwork is part of the part:

Heat-number traceability
Mill origin follows the collar through every operation to the inspection report
PMI at the door
Optical emission spectrometry on every incoming heat (50+ elements, including nitrogen, which defines these grades); no certificate taken at face value
Mechanical verification in-house
Tensile and impact testing on incoming material
Certificates with every order
Mill certs, dimensional reports, permeability verification; third-party inspection accepted and confirmed in writing before production starts. NACE MR0175 requirements supported where specified

FAQ

Q: How do you guarantee the permeability of P550 / 15-15-class alloys?

Material is procured against your permeability specification, verified by PMI on arrival, and magnetic performance is checked on the JV’s non-magnetic calibration range. State your requirement on the drawing — we verify against it, not against a brochure value.

Q: What is the longest non-magnetic drill collar you can machine?

Deep bores are production-proven to 10,000 mm; the machine envelope extends to 12,000 mm. OD turning runs to Ø800 × 8,000 mm, with slender sections to 4,500 mm handled daily.

Q: What is the difference between P550 and 15-15-class alloys in production?

Both are high-nitrogen non-magnetic austenitic alloys in our daily mix — P550 leads drill-collar volume, 15-15-class grades run in drilling components. Both carry the same traceability and verification discipline.

Q: Can you repair or modify existing collars?

Yes — narrow-gap welding of non-magnetic steels (groove angle within 2°, depth to 60 mm) and TIG/laser welding of P550 and 15-15-class alloys are in-house processes, followed by re-machining and re-verification.

Q: Do you cut API connections?

REG / NC / FH thread profiles are verified against a 24-piece API gauge suite with 64 setting plugs; API Spec Q1, 7-1 and 7-2 licenses are held by our JV Zhonghai Wantai.

See all questions we get asked →

Trademark notice: P550 is a registered trademark of voestalpine Böhler. It is used on this page solely to describe the class of material we machine; no affiliation or endorsement is implied.

P550 & 15-15-Class Non-Magnetic Alloys for MWD/LWD, in Batch Production