How Chinese Precision Alloys Are Classified
Chinese precision alloys are not classified by corrosion resistance or by strength, but by the physical property the grade is engineered to control. The grade classification standard GB/T 15018-1994 assigns a leading digit to each functional class: 1J covers soft magnetic alloys, 2J covers permanent magnetic alloys, 3J covers elastic alloys, 4J covers controlled expansion alloys, 5J covers thermocouple alloys and 6J covers electrical resistance alloys. Reading the designation therefore tells an engineer what the alloy is for before any data sheet is opened. Under that system 1J50 belongs to the soft magnetic family and 3J40 belongs to the elastic family, and the two are not interchangeable in any sense.
1J50: A Soft Magnetic Nickel-Iron Alloy
1J50 is an approximately 50% nickel-iron alloy, with the balance iron and residual elements kept low to protect magnetic performance. Its purpose is to conduct and concentrate magnetic flux with the least possible loss, so the properties that matter are high maximum permeability, low coercivity, low hysteresis loss and a high saturation induction. A nickel level near 50% sits at the point where saturation induction is still high while permeability is already strongly enhanced, compared with higher-nickel grades that trade saturation induction for permeability.
Typical characteristics include a saturation flux density of approximately 1.5 tesla, maximum permeability in the tens of thousands for correctly annealed strip, low coercivity in the order of a few tens of amperes per metre, and a Curie temperature in the region of 500 degrees Celsius. Electrical resistivity is relatively low, so the grade is normally used in thin laminations or tape-wound cores rather than in solid form, to limit eddy current losses.
Final magnetic anneal is the decisive process step. Typical practice is a high-temperature anneal in dry hydrogen in the range of about 1100 to 1150 degrees Celsius followed by controlled cooling through the ordering range. The dry hydrogen atmosphere removes carbon, sulfur and oxygen, all of which pin domain walls and degrade permeability. Cold work performed after the anneal permanently reduces magnetic softness, so any forming should be completed before the final anneal.
3J40: A Precipitation-Hardening Elastic Alloy
3J40 belongs to the elastic alloy class, whose function is to store elastic energy and return it repeatably. Elastic alloys are selected for high elastic limit, high strength, low elastic after-effect and low stress relaxation, not for magnetic behaviour. 3J40 is a precipitation-hardening type: it is supplied in a solution-treated condition that is relatively soft and formable, and it develops its strength during an aging treatment that precipitates fine second-phase particles within the matrix.
This two-stage route is what makes the grade useful. Components can be blanked, formed, wound or machined while the material is ductile, and the finished part is then aged to reach a high elastic limit with minimal distortion. Aging temperatures are deliberately moderate so that the precipitation reaction proceeds without excessive grain growth, and the treatment is normally carried out in vacuum or a protective atmosphere to prevent surface oxidation and decarburization. Where the part must also resist corrosion, a surface passivation step follows the aging treatment.
Typical 3J40 applications are instrument springs, elastic suspension elements, force and displacement sensor components, elastic diaphragms, and precision spring contacts in electrical and mechanical assemblies. In these parts the controlling requirement is usually elastic stability over the service life rather than absolute strength, and it is evaluated by measuring stress relaxation after prolonged loading at temperature.
Functional Comparison
| Item | 1J50 | 3J40 |
|---|---|---|
| Functional class under GB/T 15018-1994 | 1J, soft magnetic alloy | 3J, elastic alloy |
| Property being engineered | Permeability, coercivity, saturation induction | Elastic limit, strength, relaxation resistance |
| Nominal base | About 50% nickel, balance iron | Precipitation-hardening matrix |
| Strengthening route | None, soft magnetic anneal only | Solution treatment plus aging |
| Magnetic behaviour | Strongly ferromagnetic | Not selected for magnetic duty |
| Typical product form | Thin strip, tape-wound core, lamination | Strip, wire, small machined parts |
| Typical use | Transformers, relays, magnetic shielding, current sensors | Springs, elastic elements, sensor diaphragms |
Selection and Purchasing Notes
The two grades are specified against different delivery conditions, which is the most common source of error in procurement. 1J50 is ordered by magnetic property: initial permeability, maximum permeability, coercivity and saturation induction, each measured on a ring or strip specimen after the supplier's final anneal. 3J40 is ordered by mechanical property in the aged condition: tensile strength, yield strength, elastic limit and hardness, with the solution-treated condition specified for the forming stage. Both grades require a protective or vacuum heat treatment, and both are sensitive to surface condition, so grinding burn, oxide scale and embedded contamination must be avoided. Where a component needs both magnetic softness and spring properties, the correct answer is a hybrid design using two materials rather than a compromise grade.
Frequently Asked Questions
Q: What does the J in 1J50 and 3J40 mean?
A: The letter J marks a precision alloy in the Chinese designation system, and the leading digit identifies the functional class. Under GB/T 15018-1994, 1J is soft magnetic, 3J is elastic, 4J is controlled expansion, 5J is thermocouple and 6J is electrical resistance.
Q: Is 1J50 magnetic?
A: Yes. It is a soft magnetic nickel-iron alloy designed to be easily magnetized and demagnetized, with high permeability and low coercivity. That behaviour is the opposite of what a spring material such as 3J40 is selected for.
Q: Why is 3J40 aged after forming?
A: Aging precipitates fine particles that raise the elastic limit and strength. Forming the part in the solution-treated condition keeps it ductile and avoids cracking, and the aging treatment then hardens it with minimal dimensional change.
Q: Can 1J50 be cold worked after annealing?
A: It can be formed, but any cold work after the final magnetic anneal degrades permeability and raises coercivity. The usual route is to complete all forming, then perform the final hydrogen anneal as the last step.
Q: What heat treatment atmosphere is required?
A: Both grades require a protective atmosphere. Soft magnetic grades are annealed in dry hydrogen to remove interstitial impurities, and elastic grades are aged in vacuum or a dry inert atmosphere to avoid oxidation and decarburization of thin sections.
Q: How should the two grades be inspected on receipt?
A: 1J50 should be verified by magnetic testing of permeability or coercivity on a specimen, while 3J40 should be verified by tensile or hardness testing in the aged condition. Chemical analysis alone does not demonstrate that either grade will meet its functional requirement.
