What Is a Non-Magnetic Drill Collar?

A non-magnetic drill collar (NMDC) is a drill stem element machined from austenitic stainless steel with magnetic permeability held below about μr 1.010. Its job is simple: create a magnetically quiet zone in the bottom hole assembly so the MWD/LWD directional sensors inside it can measure the Earth’s magnetic field without interference from the steel drill string above and below.

Why ordinary collars can’t do the job

Standard drill collars are AISI 4145H Mod — a strongly ferromagnetic steel. Placed near a magnetometer, they distort the local field and corrupt the azimuth reading. Surveys taken inside a distorted field can miss the well target or, worse, fail an anti-collision check. The fix is to surround the survey instruments with enough non-magnetic length that steel-string interference falls inside survey tolerance.

The material: P550-class austenitic steel

Most NMDCs are machined from chromium-manganese austenitic stainless steel in the P550 class. The grade matters for three reasons:

1. Permeability — austenitic structure keeps μr at or below 1.010.
2. Strength — cold working brings yield strength to the 110 ksi class, comparable to 4145H, so the collar still delivers weight on bit.
3. Galling resistance — rotary shouldered connections are made and broken constantly; the grade must survive repeated make-up torque.

How permeability is verified

Each finished collar is surveyed with a calibrated permeability meter at intervals along the body and around both connections. Readings go on the inspection certificate; any local hot spot is flagged and dispositioned before shipment. This survey ships together with UT, PT, dimensional and thread-gauging reports in the documentation package.

What to include in an NMDC RFQ

• OD and ID (or the MWD tool series the collar houses)
• Overall length and connection sizes (e.g. NC50 box × pin)
• Slick or spiral body
• Permeability acceptance limit if stricter than μr 1.010
• Quantity, delivery target, and certificate requirements (API 7-1, MTC 3.1)

With a drawing — or even just these bullet points — a qualified quotation with delivery time can usually be returned within two working days. Send your RFQ here.