Question:

Can Carpenter's Invar alloy be passivated? If so, how?

Answer:

Invar is not a CRES material as it does not contain chromium to form a passive layer in air.  As such, processing the alloy through normal passivation processes used on stainless alloys would not be suggested for Invar and may result in the emission of hazardous NOx gases.

If corrosion resistance is a concern with your application, then coatings such as Cr or Ni are possible options.

Question:

Currently I am working with Carpenter’s Hiperco 50 alloy at elevated temperatures. At what temperature will I begin to affect the magnetic properties?

Answer:

Research has found that magnetic changes that occur are reversible at temperatures below 450 C. Exposures above that temperature will result in lower saturation, but will generally not affect coercive force until 730 C. 600 C is considered to be the upper-limit for reliable long term operation for the alloy.

Question:

Our customer requires 400 series stainless per ASTM F593 group 4 or 5 (Type 410 or 430). The parts are passivated in citric acid per ASTM A967-99. They have a 96 hr salt spray requirement. The threaded fasteners also have a nylon patch applied which requires heating to 400-500 F. Is it possible to meet 96 hrs in salt spray no red rust?

Answer:

Under most circumstances, Type 304L or similar alloys used to make cold headed fasteners (320HQ is a common variant on 304) will survive roughly 200 hours so long as the thread form is not terribly aggressive and the head is generally smooth. Surface roughness and surface irregularities can affect salt spray results as much as the alloy type. But 200 hours is a common test for 304 / 302HQ fasteners and they are typically capable of surpassing 200 hours to first rust.

Salt spray (ASTM B117) can be a tricky issue on which to make generalized commentary about minimum hours to first rust, etc. In general and under ideal testing conditions, Type 410 will generally show rust in 5% neutral salt spray in under a couple hours. That test is generally considered far too aggressive for Type 410. I have seen people apply coatings and mating with dissimilar metals that will give 410 over 1,000 hours, but at that point, one is not testing the alloy, they are testing a system. Type 430 will generally perform better and can approach 96 hours under ideal conditions. Fasteners with rough threads or with an aggressive thread form will most likely not attain those results. So, the short answer is that it is possible, but it is also highly likely that you will see lots occasionally not pass. Much will depend on the nylon coating you mention that is applied to the fastener.

Here are the general guidelines we give regarding salt spray testing:

Here in are general guidelines for the expected salt spray performance of various stainless steels. Again, these are not indicative of guaranteed performance and these products would not be warranted at the stated levels. This information is provided as general guidelines.

• Type 410 – Salt spray testing is too severe for this grade. Under the best possible conditions, rusting might be resisted for a short period of time (a few hours).

• Type 430 – Better than Type 410, but still borderline in terms of the severity of the salt spray test. Under the best conditions, this alloy might resist rust slightly beyond a few hours.

• Type 304 – This grade per se (not necessarily any given parts made thereof) has inherent capability of resisting rust for up to a couple hundred hours. Free-machining variants such as standard Type 303 will have slightly less capability.

• Type 316 – Capability up to several hundred hours.

Again, actual performance of parts under specific circumstances may vary from these guidelines.

Question:

I have stainless steel 304L rod parts of size dia 9.5" x 65mm. Right now, I need to bring down the permeabilty to less than 1.04. We have tried heating up the workpiece to 1050deg C, holding for 2 hours and using high pressure cool down. Do you have any suggestions to bring down the magnetic permeability?

Answer:

Type 304 has rather loose industry limits for chemistry that often results in a fair amount of ferrite in the structure of the material. Much of this is due to chemical segregation. As such, it is necessary to run longer homogenization treatments to minimize the amount of ferrite in the structure. This is typically done at the billet stage as oxidation, grain growth and cost limit the ability to do this on smaller sections. Unfortunately, there is not much you can do with the parts in question. On future orders, it would be helpful to change to a grade that would have fewer issues with ferrite content or to purchase product to a maximum ferrite/permeability specification. Type 316 will typically have less ferrite content as will Cr-Mn-N 200 series stainless steels.

 Question:  What temperature do you suggest to use for stress relieving Kovar® alloy?

Answer:  Carpenter’s Kovar® alloy may be stress relieved at 600 C and then air cooled. It is best to do this in a protective atmosphere or vacuum to avoid oxidation. A full anneal performed according to our technical data sheet should be done to gain the full potential of the alloy, but the stress relieving treatment may be used to remove residual stresses developed during fabrication.

Question:    

Can 304 and 304L be welded together? Will there be any corrosionor any other issue when welding the two types of steel together?

Answer:    

304 and 304L can be welded together. However, it will be necessary to impart a post weld anneal. If you do not, the 304 material in the heat affected zone will be sensitized and will be prone to corrosion and poor mechanical properties.

Please refer to more detailed information on welding of stainless steels starting on page 73 of Carpenter’s Guide to Selecting Specialty Alloys booklet. Download the booklet from our Product Literature request form on cartech.com.

Question:  Can you please tell me what your standard tolerance is for your 7/16” Hex Project 70+ Type 316 material.  Please let me know what the Plus and Minus is.

Answer:  The standard tolerance for .4375” hex is +.000” / -.003”.  Standard tolerances for hex products are always all on the minus side to ensure that wrench dimensions are not exceeded.