ASTM A342 Magnetic Permeability Testing of 316LN Stainless Steel: Why Inspection Location Matters

Introduction

Magnetic permeability is an important property for 316LN stainless steel used in nuclear power, MRI systems, scientific instruments, and other low-magnetic applications.

Recently, we encountered a case involving a 65 mm thick 316LN stainless steel plate. The material met the specified permeability requirements after solution annealing, but higher values were later detected at the flame-cut edges

This raised an important question: Does ASTM A342 specify where magnetic permeability should be measured?

Project Background 

The material was supplied with a magnetic permeability test report issued by the steel mill after solution annealing.

During the customer's inspection, measurements taken on the original plate surface met the requirement, while          measurements at the flame-cut edges showed higher values.

Since all samples originated from the same heat and the same plate, the difference could not be attributed to            material batch variation.

What Does ASTM A342 Specify?

ASTM A342/A342M is a standard that provides methods for measuring magnetic permeability of weakly magnetic materials.

The standard defines:

• Test methods

• Measurement procedures

• Instrument requirements

However, ASTM A342 does not specify:

• Measurement locations

• Surface preparation requirements

• Number of test points

• Whether flame-cut edges should be included in acceptance testing

This is often overlooked in project specifications.

Sample Verification

To better understand the discrepancy, six specimens were retained and tested.

The samples included:

1.Supplier arbitration samples

2.End-user retained samples

3.Independent third-party laboratory samples

All specimens were taken from the same plate.

Figure 1. Retained samples from different parties

Test Results

The measured permeability values varied depending on the inspection location and surface condition.

Measurements performed on machined plate surfaces remained relatively consistent, while higher values were observed at flame-cut edges.

This indicates that inspection location can significantly influence magnetic permeability results, even when the same material and test method are used.

Table 1. Magnetic permeability results of retained samples

Key Takeaway

One of the most important lessons from this project is that specifying ASTM A342 alone may not be sufficient for critical low-magnetic applications.

When magnetic permeability is a key acceptance criterion, project documents should clearly define:

• Measurement location

• Surface condition

• Acceptance criteria

• Whether cut edges are included in inspection

Without these details, different parties may obtain different results while following the same testing standard.

Conclusion

This 316LN thick plate project demonstrates that ASTM A342 provides a recognized testing method, but it does not specify where measurements must be taken.

In our case, all samples came from the same heat and the same plate, yet different inspection locations produced different permeability readings.

For low-magnetic stainless steel applications, clearly defining inspection requirements is just as important as specifying the material itself.