Carbon Steel Stud Bolt with Nut for Flange

author: Qishine Industry

2023-09-22

One-Minute Overview

For pressure flanges the standard set is an A193 B7 stud with an A194 2H heavy hex nut.
The stud runs through both flanges; two nuts pull them onto the gasket with even load.
Match the bolt grade to the flange class and the line temperature, not just to price.
Torque in a cross pattern to the gasket spec and re-check after the first heat cycle.

Carbon Steel Stud Bolts with Nuts for Flanges at a Glance

Carbon steel stud bolts with two heavy hex nuts are the standard fastening set that pulls a flanged pipe joint together. For pressure flanges the usual pairing is an ASTM A193 B7 stud with an A194 2H nut. This guide covers the common grades, how the stud and nuts clamp the joint against a gasket on a matched flange, how to match the bolt to the flange class and temperature, coatings, and the torque steps that keep the seal tight.

What the Grade Marks Mean

Carbon steel flange studs are graded by ASTM; the nut must match the stud:

A193 B7 + A194 2H: the default for pressure flanges to about 450 deg C (840 deg F).
A193 B16 + A194 7: high-temperature flanges above 450 deg C.
A307 + A563 or 2H: low-strength, ambient, non-pressure joints.
Plain carbon steel rusts - pick a coating or stainless for wet or corrosive service.

How the Stud Clamps the Joint

The stud is a bar threaded at both ends with a plain shank between. It passes through both flange bolt holes; one nut seats against the back flange, the other is tightened to pull the faces together and compress the gasket. Because there is no head, the load spreads evenly and the stud can be changed without freeing the pipe.

Match the Bolt to Flange Class and Temperature

Flange duty	Stud	Nut
Class 150, ambient	A193 B7 or A307	A194 2H / A563
Class 300-600, to ~450 deg C	A193 B7	A194 2H
Above 450 deg C	A193 B16	A194 7
Sour (H2S) service	A193 B7M	A194 2HM

Coating and Corrosion Care

Choose the finish by the environment:

Plain (black) finish for dry, indoor, or painted service.
Zinc plate or hot-dip galvanize for outdoor; galvanizing adds thickness, so torque values change and threads may need cutting after coating.
PTFE or Xylan coating for sticky, corrosive, or frequent-open joints.
For stainless or alloy flanges in wet service, step up to a stainless stud to avoid galvanic corrosion.

Installation: Torque and Sequence

Clean the threads and the nut faces, start both nuts by hand, then torque in a cross pattern in three passes (30, 60, and 100 percent) to the value from the flange or gasket spec. Use a compatible anti-seize on coated or stainless parts. Do not over-torque - a stretched B7 loses preload and the gasket relaxes. Re-torque after the line reaches temperature, because the gasket and the joint settle.

Common Mistakes

The failures we see most often:

Mixing nut grades - a standard hex nut is too thin for a B7 stud and can strip.
Using the wrong stud length so the nut has too few threads engaged.
Over-torquing in one pass, which yields the stud or crushes the gasket.
Reusing nuts that have galled or worn threads.

FAQ

What grade stud bolt is used for flanges?

For pressure flanges the standard is an ASTM A193 B7 stud with an A194 2H heavy hex nut; A307 is for low-strength ambient duty.

Stud bolt or hex bolt - which is better for a flange?

A stud runs through both flanges so the load is even and the stud is easy to replace; a hex bolt is fine for light, non-critical joints.

How long should the stud be?

Long enough to seat both nuts with a few threads protruding past the top nut. Size it to flange thickness plus gasket plus both nut heights.

What torque should I use?

The value from the flange or gasket specification, applied in a cross pattern. Re-torque after the line heats up.

Can I reuse the nuts?

Only after inspection - discard any that have galled or worn threads. Many specs allow sound nuts to be reused.

What is B7M?

B7M is the lower-hardness version of B7 for sour (H2S) service; pick it when the plant's NACE requirement calls for it.

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