Abstract:

The effects of embrittlement relief (bake) delays, bake temperatures and bake times were assessed using cadmium plated notched tensile specimens. Short delay times into the bake oven were found to be very important in obtaining freedom from embrittlement. Extended bake times assist in alleviation of embrittlement potential. Bake temperatures above industry standards appear to be very beneficial in counteracting potential embrittlement situation. A transition point of permanent embrittlement appears to occur with bake delay times of 16+ hours.

Introduction:

Current industry practices and procedures concerning embrittlement relief (baking) are the result of several decades of research, testing discussion and compromise. Embrittlement relief practices have been perceived by the industry as time consuming and expensive, such that many times only the absolute minimum baking schedules are utilized. The technical sophistication of steel alloys available today are a far cry from those present in the 1960's, with higher strength, lower weight steel components being the norm for aircraft and aerospace structures. This trend toward very high strength ferrous alloys can also mean lower safety margins as it pertains to metal finishing processes. Current bake delay times, actual bake times and temperatures were evaluated utilizing industry standard high strength steel notched samples. Alternate methods for the alleviation of embrittlement in marginal situations were explored also.

Experimental Procedure:

The test samples utilized for this study were produced from 4340 low alloy steel at 260-280 ksi fabricated per ASTM F519 TYPE la (1). This is the standard for the aircraft chemical processing industry and is widely specified in most Federal, Military, AMS and ASTM plating specifications. A total sample population of 110 bars was allocated for the study with 5 bars utilized per test parameter. Samples for this test were subjected to 200% inspection per ASTM F519 requirements. All test samples came from the same manufacturing lot. Test samples were stressed on standard sustained tensile load stress rupture machines per ASTM F519 loading requirements at 75% of notched ultimate tensile strength. Loading accuracy was determined before and after the test per ASTM E-8 and ASTM E292 requirements. Time calibration was provided from NIST via WWV broadcasts.

Highly embrittling plating conditions per ASTM F519 Annex Treatment A (Cadmium) were utilized, with sample set plating performed using 5 samples at a time. Controlled laboratory plating conditions utilizing digital current control occurred. Current density control at the cathode accurate to +/- 0.1 Amp. was utilized. Plating solutions for the entire test program were prepared prior to laboratory plating.

Because the effects of baking efficiency were not desired to be introduced in this program, the following schedule was used:

1) Cadmium plate 2) Strip in NH₄NO₃ to bare steel, 30 minutes prior to entry into bake oven 3) Bake / Test

The embrittling nature of cadmium plating can change radically depending on the plating condition, especially brightener content. Highly brightened baths, utilizing low current densities and long plating times, promote embrittling subsequent baking schedules for relief may or may not be effective due to the lower porosity nature of the plating. Numerous contractor plating processes, such as Boeing Specification BAC 5804(2), control the cadmium plating embrittling tendencies utilizing the porosity meter technique.

Ammonium nitrate is an alkaline method of stripping cadmium plate widely utilized in the aircraft industry and of itself does not contribute to embrittling tendencies. Since the test bars were stripped to bare steel and quickly compressed air dried, a uniform optimized surface condition was present on all test samples prior to all bake and test program parameters.

Other metallic plating methods could have been employed for this study, i.e. chromium, nickel, etc.; however, it was deemed optimal to utilize the highly embrittling cadmium bath with a subsequent strip to produce the most consistent test conditions.

Cadmium stripping was accomplished in a NH₄NO₃ bath at room temperature. Stripping time was 30 minutes. A tabular test matrix for this investigation is presented in Table I.

Results:

All results for this investigation are tabulated as time to fracture and are presented in Tables II & III. Graphical representation of the data is presented in Figures A and B.

The three parameters for this program, in order of investigation importance were once again 1) bake delay into the oven, 2) bake temperature and 3) bake time.

Four pronounced conclusions can be obtained from the available data:

1) Relatively immediate transfers into the bake oven after plating (30 minute delay due to stripping) produced no failures.

2) A standard relief schedule of 4 hour delay with a 375 degrees F bake at 8 hours still resulted in a failure rate of 20% (one of five).

By extending the bake time to 23 hours, no failures occurred. Also, by going to the slightly elevated bake temperature of 425 degrees F, no failures occurred at either the 8 or 23 hour bake times.

The statistical significance of 1 in 5 failures was intriguing to this investigator. The data point was repeated with 5 more bars processed in identical fashion. Two failures were noted in this secondary test; one bar at 151 hours and the second at 203 hours. (The test was allowed to run over 200 hours due to laboratory scheduling interferences).

3) Allowing the delay time to extend beyond 4 hours before entry into the bake oven produced failures with both 8 hour bake times and 23 hour bake times. Only by going up to the elevated temperature of 425 degrees F did the effects of an 8 hour delay become alleviated.

4) With bake delay times of 16 and 24 hours, no remedial attempts to completely alleviate embrittlement proved successful; i.e. 8 and 23 hour bake times at 375 degrees F and 425 degrees F all produced some failures.

Discussion:

The metallurgical mechanisms of hydrogen embrittlement of high strength steel have been heavily studied, discussed and argued about through the years. It is not within the scope of this study to expound on new or old theories on the solid state physics of this phenomena. It is rather to assess, through controlled experimental means, possible remedial techniques to alleviate embrittling effects.

Delay times into the embrittlement relief or baking oven appear to be standardized today at a 4 hour maximum (4). Several exceptions exist in the aircraft industry; i.e. Bell Helicopter (3) requires a maximum of 1 hour from plating to bake oven, and one European aircraft concern requires immediate transfer into the baking oven from rinse and blow dry. Since hydrogen embrittlement mechanisms are diffusion controlled, the effects of time delay into bake are very important. Residual and applied stresses with the steel component are primary drivers for hydrogen interaction, and thus the higher strength steels are more sensitive to transfer delay times. It would be of interest to develop test scenarios utilizing test samples of different strength/residual stress levels; however, this was not possible for this program.

The second test parameter of importance during this investigation is that of subsequent baking times after entry into the bake oven. Once again, since the effects of embrittlement can be greatly reduced by numerous diffusion variables, time at temperature definitely assist in embrittlement relief.

The third parameter of interest for this current program is that of baking temperature. The aircraft industry has standardized with two basic temperature schedules. The first is the higher bake, 375 degrees F utilized for most steel components, and the second is a lower 275 degrees bake temperature for alloys sensitive to metallurgical tempering effects, i.e. carburized, induction hardened or ball beating steels. Differences do exist from contractor to contractor; once again, Bell Helicopter calls for slightly higher temperatures than the norm at 385 degrees F for the standard bake schedule (3). Since solid state interstitial diffusion proceeds via an exponential type relationship, the effects of higher temperatures affect the actual amount of hydrogen movement and removal in radically enhanced fashion.

The rational for a 275 degrees bake temperature for surface hardened and bearing steels is self-evident. However, the standard 375 degrees bake temperature rationale is somewhat cloudy in its origin. It appears traceable to the late 1950's and early 1960's when high strength steel alloys for aircraft structural and landing gear use were based upon low alloy steels, i.e. 4340, 4330, etc. and pushed to their strength limits via low tempering temperatures. Today, most landing gear steels are based upon modified 300M chemistries and usually tempered at 575 degrees F such that higher baking temperatures could be utilized without concern for tempering effects on the base steel. A second variable that should be considered is the effect of higher temperatures on shot peened steel substrates. However, current industry standards allow post shot peened temperature exposure up to and including 475 degrees F for steel components (5).

In addition, temper embrittlement of some low alloy steels should not be of concern since this effect (sometimes known as blue brittle) does not manifest itself at temperatures below approximately 550 degrees F (6).

The initial results of studying fast bake transfer times proved to follow what could be expected via standard metallurgical diffusion mechanisms. By quickly transferring into the bake oven, the opportunity for nascent hydrogen to begin its inward migration is halted. Regardless of what theories of embrittlement one subscribes to, by denying nascent hydrogen the opportunity to diffuse inward, embrittlement can be eliminated. The data reinforces the position of Bell Helicopter and some European prime contractors concerning bake delays.

The second test program parameter of a 4 hour delay into bake produced the initial program failures. Utilizing the standard 8 hour bake time at the standard 375 degrees F bake temperature, a failure rate of 20% was experienced. This is important for the aircraft and aerospace industry to review the adequacy of this schedule as applicable to very high strength steels. Lower strength steel alloys should probably exhibit less tendency towards embrittlement failures with a 4 hour delay/8 hour bake. It is interesting to note that extending the bake time to 23 hours produced no failures and also that 425 degrees F bake temperatures alleviated embrittlement at both the 8 and 23 hour bake times. A 375 degrees F 23 hour bake schedule producing no failures reinforces the baking requirements in place now as governed in the Federal Specification QQ-C-320 (3) for chrome plate.

The retests performed for this program parameter produced results slightly different than initially obtained, but nonetheless reinforced the previous conclusions.

The third test program parameter of an 8 hour delay time into bake shows failure rates of concern, i.e. 40% at an 8 hour bake time and 20% at the 23 hour bake time. However, the elevated bake temperature of 425 degrees F did alleviate embrittling effects associated with the 8 hour delay. It is relevant to comment here on probable metallurgical mechanisms occurring. It appears that the thermal energy present at 375 degrees is not sufficient to alleviate embrittlement regardless of bake time. One could conclude that some form of meta-stable hydrogen damage has occurred at this point either in the form of grain boundary films or possible re-combined hydrogen. At any rate, extended time did not significantly eliminate embrittlement. On the other hand, by raising the bake temperature to 425 degrees F, it appears the energy barrier of this meta-stable state has been breached allowing embrittlement relief either by hydrogen dissociation or cleating of grain boundary films.

The fourth test program parameter of a 16 hour bake delay produced some of the most interesting results of the program. Failure rates of 80% were experienced at the standard 375 degrees F for 8 hour bake schedule, 60% for the 375 degrees for 23 hours schedule and even failures at the 425 degrees F bake temperature. The obvious conclusion is that at a delay time of 16 hours, no recovery from embrittlement was experienced. Whether extensive micro-crack development has already begun before entry into the bake oven or possibly a binary type meta-stable recovery curve is present is not known. Possible subsequent investigations of the 16 hour delay at 475 degrees F could prove the possibility of a binary recovery curve.

The Fifth and last test program parameter of a 24 hour delay time produced failures with all schedules. The metallurgical mechanisms as noted for the 16 hour delay probably tend now towards more definite micro-crack development, irreversible in nature.

The presence of a semi-porous or non-porous metallic plating on the surface of a part or component will of course influence the efficiency of any baking operation. The present investigation sought a near perfect, consistent surface condition (i.e. no plate present) in order to evaluate bake out capabilities. Therefore, the data generated from this program should be considered as occurring under optimal conditions. Highly brightened cadmium plate, zinc or nickel plating baths can very well produce a surface condition such that short delay times, extended bake times and elevated temperature bake schedules may have little impact on embrittlement relief.

1) Short delays into the embrittlement relief or bake oven are very important and should not be taken lightly in the processing of high strength steel parts.

2) Extended bake times assist in the alleviation of embrittlement if delay times are kept low.

3) Raising the bake temperature 50 degrees F appears to be a powerful method in alleviating embrittlement when presented with delay times of 8 hours nominal.

4) A point of permanent embrittlement damage appears to begin with delay times of 16 hours and beyond. The metallurgical mechanism of embrittlement most probably changes with delay dwell times in the range of 16+ hours.

5) The embrittlement effects noted during this program could be expected to moderate with lower strength steel samples or components. Lowering the residual stress driving force for hydrogen migration and interaction can be expected to also lower the failure rate.

Acknowledgment:

The author gratefully acknowledges the assistance of Richard G. Green of Green Specialty Service, Ft. Worth TX, in providing all test samples for this investigation.

References:

(1) ASTM F519 published by the American Society for Testing and Materials, 1916 Race Street, Philadelphia, PA 19103

(2) Boeing Commercial Airplane Co., Process Specification BAC 5804 "Low Hydrogen Embrittlement Cadmium-Titanium Alloy Plating"

(3) Bell Helicopter Textron, Process Specifications BPS 4006, et. al.

(4) Federal Specifications:

QQ-N-290 Nickel Plating
QQ-C-320 Chrome Plating
QQ-P-416 Cadmium Plating

Military Specifications:

MIL-C-26074 Electroless Nickel
MIL-G-45204 Gold Plating
MIL-C-13924 Black Oxide Coatings
DOD-P-16232 Phosphate Coatings

(5) Military Specification MIL-S-13165, "Shot Peening of Metal Parts"

(6) Military Handbook MIL-HDBK-5 "Metallic Materials and Elements for Aerospace Vehicle Structures"

¹ Registered Professional Engineer, Omega Research and Engineering, Southlake, TX 76092 Submitted for presentation, First International Technology Transfer Conference, Hydrogen Embriulement of Fasteners, Denver CO, May 1995.

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