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Magnetism

Why is stainless steel sometimes magnetic?

Magnetic Properties of Stainless Steels

Source: Carpenter Stainless Steels (1999). Magnetic Properties of Stainless Steels pp. 295-296.

The magnetic behavior of stainless steels varies considerably, ranging from paramagnetic (nonmagnetic) in fully austenitic grades to hard or permanent magnetic behavior in the hardened martensitic grades. Stainless steels have not found widespread use solely as magnetic materials since their magnetic capability is almost always inferior to conventional magnetic materials. However, there are circumstances and applications where the magnetic (or nonmagnetic) behavior can significantly influence fabrication and use of these alloys.

Austenitic (nonmagnetic) Stainless Steels

All austenitic stainless steels are paramagnetic (nonmagnetic) in the fully austenitic condition as occurs in well-annealed alloys. The DC magnetic permeabilities range from 1.003 to 1.005 when measured at magnetizing forces of 200 oersteds (16KA/m).The permeability increases with cold work due to deformation-induced martensite, a ferromagnetic phase. For certain grades such as 302 and 304, the increase in magnetic permeability can be appreciable, resulting in these grades being weakly ferromagnetic in the heavily cold-worked condition. The susceptibility of a particular grade to becoming ferromagnetic when heavily cold worked depends on the stability of the austenite, which, in turn, depends on chemical composition and homogeneity. This is described in the article 'Stability of Austenite in Stainless Steels' by C.B. Post and W.S. Eberly, published in 'Transactions of the American Society for Metals,' volume 39, (1947), pages 868 to 890 inclusive.

The effect of cold work on magnetic permeability is illustrated for several austenitic stainless steels in figure 1. The relationship between ultimate tensile strength and magnetic permeability is shown in Figure 2. It will be seen that the rise in permeability correlates well with the increase in tensile strength or work-hardening behavior, which is another measure of austenite stability. The differing performance between grades is a reflection of their composition. In particular, nickel increases the work-hardening rate and the rate of increase of magnetic permeability. Consequently, the higher nickel grades, such as Carpenter No.10 exhibit lower magnetic permeabilities than the lower nickel grades, such as Project 70R stainless Type 304 when cold worked in equivalent amounts. The high-manganese, high nitrogen alloys , such as Carpenter 18 Cr-2Ni-12Mn stainless, are also noted for maintaining low permeability after heavy deformation.

The magnetic permeabilities achievable in austenitic stainless steels are very low as compared to conventional magnetic materials such as silicon-iron alloys. Consequently, it is their non-magnetic behavior which is more of concern. Certain uses such as housings and components for magnetic detection equipment used for security, measuring and control purposes require that the steel be nonmagnetic since the presence of even weakly ferromagnetic parts can adversely affect performance. Unless the austenitic stainless steel parts are used in the annealed condition and are not subjected to deformation during use, a higher nickel grade would be a prudent choice assuming it offered the appropriate corrosion resistance and strength. It should be noted that for a given grade the magnetic permeability can vary significantly depending on the particular chemistry and degree of cold work of the steel. Often a particular lot of an 'unstable' grade such as Project 70 stainless Type 304 can perform satisfactory. If the magnetic permeability of an austenitic stainless steel is of particular concern, it can be measured by relatively simple means as described in ASTM Standard Method A342. This ASTM method is being revised.

Ferritic Stainless Steels

Ferritic stainless steels are ferromagnetic and have been used as soft magnetic components such as solenoid cores and pole pieces. Although their magnetic properties are not generally as good as conventional soft magnetic alloys, they are successfully used for magnetic components which must withstand corrosive environments. As such, they offer a cost-effective alternative to plated iron and silicon-iron components. In addition, the relatively high electrical resistivity of ferritic stainless steels have resulted in superior AC performance. Soft magnetic properties, i.e. high magnetic permeability, low coercive force (Hc) and low residual induction (Br), depend strongly on alloy chemistry, particularly impurities such as carbon, sulfur and non metallic inclusions, and stress due to cold working. Magnetic permeability decreases and the coercive force increases, i.e., the behavior is less magnetically soft, with increasing amounts of impurities and stress. Consequently optimum magnetic performance is obtained with well annealed, high purity alloys. Carpenter produces two grades of ferritic stainless steel, Carpenter 403F Solenoid Quality and Carpenter 403FR Solenoid Quality, for consideration in soft magnetic alloy applications. These two grades are melted and processed for consistent magnetic properties while offering corrosion resistance similar to that of Type 430F.

Even if a ferritic stainless steel is not being used as a magnetic component, its magnetic behavior can be of significance to fabrication and use. Annealed ferritic Stainless steels exhibit soft magnetic behavior which means they do not have the ability to attract other magnetic objects when removed from an externally applied magnetic field. Cold working, however, increases the corrective force (Hc) of these steels changing their behavior from that of a soft magnet to that of a weak permanent magnet. If parts of cold worked ferritic stainless steel are exposed to a strong magnetic field such as occurs in magnetic particle inspection, they can be permanently magnetized and therefore able to attract other ferromagnetic objects. Apart from possibly causing handling problems, they are able to attract bits of iron or steel which will, if not removed, impair the corrosion resistance. It is therefore prudent to either electrically or thermally demagnetize such parts if they have been subjected to a strong magnetic field during fabrication. Magnetic properties of some ferritic stainless steels are listed in the table below.

Martensitic and Precipitation Hardenable Stainless Steel

All martensitic and most precipitation hardenable stainless steels are ferromagnetic. Due to the stresses induced by the hardening transformation, these grades exhibit permanent magnetic properties. If magnetized in the hardened condition for a given grade, the coercive force tends to increase with increasing hardness rendering these alloys more difficult to demagnetize. Although not used as permanent magnets to any significant extent, the previously mentioned potential difficulties of hardened ferritic stainless steels also apply to these steels. Magnetic properties of some martensitic steels are also shown in the table below.

Table 1:

Magnetic Properties of Some Ferritic and Martensitic Stainless Steels
Grade Condition Rockwell Hardness Maximum Relative
Permeability
Coercive Force (Hc)
Oersteds A/M
Type 410
(Martensitic)
A
H
B 85
C 41
750
95
6
36
4800
2900
Type 416
(Martensitic)
A
H
B 85
C 41
750
95
6
36
480
2900
Type 420
(Martensitic)
A
H
B 90
C 50
950
40
10
45
800
3600
Type 430F
Solenoid Quality
(Ferritic)
A B 78 1800 2 160
Type 430FR
Solenoid Quality
(Ferritic)
A B 82 1800 2 160
Type 440B
(Martensitic)
H C 55 62 64 5100
Type 446
(Ferritic)
A B 85 1000 4.5 360
Above data determined on round bars 0.375” (9.53 mm) to 0.625” (15.88 mm) per ASTM A341-Fahy permeameter.
A - fully annealed , H - heat treated for maximum hardness.

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