Significance and Use
The hot salt test as applied to metals is utilized as a secondary design consideration indicator, as cracking has been shown to occur in laboratory tests simulating possible service conditions. Although limited evidence exists linking this phenomenon to actual service failures, cracking under stress in a hot salt environment should be recognized as a potential design controlling factor.
The hot salt test is not to be misconstrued as being related to the stress corrosion cracking of materials in other environments. It is considered solely as a test in an environment that might be encountered in service.
Because hot salt cracking under stress is considered a secondary design consideration and service failures have not been attributed solely to this phenomenon, manufacturing processes will be optimized or alloying changes will be made only after consideration is given to primary design factors such as creep resistance of a given high temperature alloy. The usefulness of the test lies rather in limiting maximum operating temperatures and stress levels or categorizing different alloys as to susceptibility, or both, if it is found that hot salt damage may accelerate failure by creep, fatigue, or rupture.
Finally, the test does not lend itself to the utilization of pre-cracked specimens because cracking reinitiates at any salt-metal-air interface, resulting generally in many small cracks which extend independently. For this reason, specimens that are recommended for utilization in routine testing are of the smooth specimen category.
Scope
1.1 This practice covers procedures for testing metals for embrittlement and cracking susceptibility when exposed under stress to a hot salt environment. This practice can be used for testing all metals for which service conditions dictate the need for such information. The test procedures described herein are generally applicable to all metal alloys; required adjustments in environmental variables (temperature, stress) to characterize a given materials system should be made. This practice describes the environmental conditions and degree of control required, and suggests means for obtaining this desired control.
1.2 This practice can be used both for alloy screening for determination of relative susceptibility to embrittlement and cracking, and for the determination of time-temperature-stress threshold levels for onset of embrittlement and cracking. However, certain specimen types are more suitable for each of these two types of characterizations.
Note 1This practice relates solely to the performance of the exposure test. No detailed description concerning preparation and analysis of specimen types is offered. However, the optimum sample design may be one that uses the same type of stress encountered in service loading situations. Standards describing principal types of stress corrosion specimens, their preparation, and analysis, include Practices G 30, G 38, and G 39.
This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.
