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Magnetic Particle Testing of High Tensile Parts
 Used in Aerospace Applications

by Robert Potter*

 

In this article the author draws our attention to the need for understanding material properties, processes and geometry when searching for discontinuity location. High tensile materials are a good example of this. When developing a magnetic particle testing technique, it is important to carefully review geometry, material conditions and other information relevant to successful location of all discontinuities. The article serves as a reminder to review technique sheets for atypical conditions and resulting discontinuity locations before performing a test. Bruce Crouse Contributing Editor

Figure 1-3

Introduction
One of the primary challenges of magnetic particle testing of aerospace parts is having a command knowledge of the various materials and product forms involved. While most parts subject to magnetic particle testing support the aircraft structure and require mechanical properties that allow flex and bending while in service, several parts serve as wear surfaces and require high tensile properties. Each of these two groups of parts has unique mechanical properties and discontinuity mechanisms.

Materials
Most of the steels used in aircraft structures consist of low alloy/medium carbon steels (UNS G41300, UNS G41400) and the precipitation hardened stainless steels (UNS S13800, S15500, S17400 and S17700). A majority of the indications identified in magnetic particle testing are inherent discontinuities. Low alloy steels commonly have ferrite stringers, laps, folds, nonmetallic inclusions and a variety of indications oriented along the long axis of the part (Figure 1a). Precipitation hardended steels also possess indications oriented along the long axis and include alloy segregation, ferrite stringers (Figure 1b) and nonmetallic inclusions (ASM International, 1976).

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Knowledge of material type
 and heat treat condition is imperative
 to improve the probability of detection.

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The heat treat of the low alloy/medium carbon steel results in a typical Rockwell C hardness of 35 RC and a tensile value of approximately 1103 MPa (160 000 lb/in.²). These values support the design function for strength and elasticity. The yield strength and elongation are typically 951 MPa (138 000 lb/in.²) and 14%, respectively (Boyer, 1999). The precipitation hardened steels, unlike other steels, achieve both high strength and corrosion resistance using age tempering techniques. Precipitation hardened steels are classified as either martensitic or semiaustenitic. Typical mechanical values for precipitation hardened steels are provided in Table 1.

Table 1 Mechanical properties of precipitation hardened steels
Steel	Tensile Strength	Hardness
UNS S15500 martensitic	1034 MPa (150 000 lb / in.2)	38 Rc
UNS S17400 martensitic	1379 MPa (200 000 lb / in.2)	44 Rc
UNS S13800 martensitic	1379 MPa (200 000 lb / in.2)	45 Rc
UNS S17700 martensitic	896 MPa (130 000 lb / in.2)	85 Rc

Product Forms
Aircraft structural parts are primarily machined from wrought materials. These materials receive primary and secondary processing by undergoing a series of rolling operations to achieve the desired shape, microstructure and mechanical properties (Figure 2a). As a result, the discontinuities associated with the wrought process principally run in the long grain direction and are detected by magnetic particles on the short longitudinal or longitudinal transverse face (Figure 2b). One of the most significant errors that magnetic particle inspectors can make is to become complacent with the common discontinuity mechanism and calibrate their test equipment to only concentrate on the primary faces described. This is especially true when testing parts that possess high tensile values.

One such material is UNS G52986. This material is a high carbon/low alloy material that is used for bearing applications. The material achieves its high tensile strength by undergoing a heat treat followed by a rapid quench. The resulting mechanical properties yield a tensile value of approximately 2241 MPa (325 000 lb/in.²) and a hardness of 58 R_C. Several discontinuity mechanisms can affect the part undergoing the quench process. First, the material is subject to a rapid cooling. Any area of uneven thickness will result in the thinner area cooling first. The thicker area will cool more slowly and will pull the molecules of the thinner cooled area toward the area that is last to fully solidify (Figure 3a). This may yield quench cracks in or near the thickness transition area. The other key component is notch effect. If the part has sharp radii, stress can result during part cooling. Again, the thinner area may be pulled toward the thicker member and cracks can originate.

Lastly, these parts are machined like all wrought materials with the principal stresses running along the long axis. When the parts possess multiple thicknesses or are subject to notch effect, discontinuities are subject to originate in the short transverse face (Figure 3b). This fact requires due diligence from the magnetic particle inspector. To improve the probability of detecting discontinuities in high tensile parts, the following is recommended:

• following the application of the particle suspension and the two successive magnetic shots, carefully observe the part during the 15 s dwell time (ASTM International, 2001)

• carefully test the part on all possible faces (remember that stress cracks are prone to originate on the short transverse face)

• because the parts are highly tensile with high hardness, the permeability values will be lower than for other low alloy steels - if the technique is qualified using a hall effect probe, it is recommended that the magnetic force value be on the high end, 5 to 6 mT (50 to 60 G), though this may result in additional background noise.

 

Conclusion
It is important to perform research on parts prior to establishing a test strategy. High tensile parts possessing discontinuities unique to the product form can easily be missed. Knowledge of material type and heat treat condition is imperative to improve the probability of detection.

 

References
ASM International, Heat Treating, Cleaning and Finishing, 8th edition, Vol. 2, Materials Park, Ohio, ASM International, 1976.

ASTM International, ASTM E-1444: Standard Practice for Magnetic Particle Examination, West Conshohocken, Pennsylvania, ASTM International, 2001.

Boyer, Howard E., Practical Heat Treating, Materials Park, Ohio, ASM International, 1999.

 

* Metal Finishing Company, 1329 S. McLean Blvd., Wichita, KS 67213; (316) 267-7289; fax (316) 267-1861; e-mail <bobpotter@metalfinishingco.com>.

 

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