Hydrogen embrittlement sustained load tensile testing is the industry-standard method for verifying that electroplating, coating, pickling, and cleaning processes will not cause delayed brittle fracture in high-strength steel components. Based primarily on ASTM F519, the test holds sharply notched specimens under a constant tensile load—typically 75% of the notched tensile strength—for up to 200 hours to detect hydrogen-induced delayed cracking. This article explains the testing principle, specimen types, test items, procedures, acceptance criteria, and typical applications across the aerospace, automotive, and fastener industries.
Technical Overview
Hydrogen embrittlement sustained load tensile testing (also called sustained load testing, SLT, notched sustained-load tensile testing, or delayed fracture testing) is a mechanical qualification test used to evaluate the susceptibility of high-strength steels to internal hydrogen embrittlement (IHE). During processing operations such as acid pickling, cathodic cleaning, and electroplating, atomic hydrogen is generated on the metal surface and absorbed into the steel. If this hydrogen is not effectively removed by a subsequent relief bake, it can diffuse toward regions of high triaxial tensile stress and cause subcritical, intergranular cracking under sustained loading.
The defining feature of this test is that failure is time dependent. A conventional rapid tensile test may show full strength even though the material is embrittled, because hydrogen requires time to migrate to the stress concentration site. In the sustained load configuration, a circumferentially notched round specimen—designed to create a severe stress concentration and triaxial stress state—is held at a constant fraction of its notched tensile strength for an extended dwell period. Embrittled specimens fracture during the hold; sound specimens survive without cracking.
The principal governing standards include ASTM F519 (mechanical hydrogen embrittlement evaluation using notched specimens), ASTM F1940 (process control verification for plated or coated fasteners), ASTM B850 (post-coating hydrogen embrittlement relief of steels), and AMS 2759/9 (hydrogen embrittlement relief treatment). Related environmental hydrogen embrittlement practices are covered by NACE MR0175 / ISO 15156, NACE TM0177, and ASTM G142, while fastener-specific checking requirements appear in EN ISO 4042 and the GB/T 3098 series. Accredited third-party laboratories routinely perform this test for process qualification and lot acceptance.
Testing Samples
Notched Round Bar Specimens (ASTM F519 Type 1 Series)
The most widely used configuration is the notched cylindrical specimen, exemplified by the Type 1a "LAP" (lot acceptance procedure) specimen. It features a circumferential 60-degree V-notch with a stress concentration factor (Kt) in the range of approximately 3.3 to 5.0 depending on the variant. Specimens are typically machined from AISI 4340 (or 4130, 300M, and other specified alloys), quenched and tempered to a high-strength condition commonly in the range of about 1790 to 1930 MPa (260 to 280 ksi), with hardness verified by the testing laboratory. Notch machining and grinding are strictly controlled to avoid introducing residual stress or burns that would distort results.
Actual Production Parts and Fasteners
ASTM F519 also permits testing of actual hardware, including structural fasteners, pins, studs, and other production components that have passed through the complete production coating cycle. High-strength fasteners of property class 10.9 and 12.9, aerospace bolts to NAS and MS specifications, landing gear hardware, and hardened shafts are frequently evaluated in this manner, either under direct axial sustained load or under a sustained preload in a fixture.
Process-Control Witness Specimens
For fastener and plating process control under ASTM F1940, notched witness specimens are processed through the same production line—cleaning, pickling, plating, conversion coating, and baking—as the parts they represent. This verifies the hydrogen embrittlement safety of the entire process chain rather than a single component geometry.
Precharged Specimens for Material Screening
In research and material qualification programs, specimens may be cathodically precharged with hydrogen in a controlled environment before loading, in accordance with ASTM G142, to screen material grades, heat treatment conditions, or coating systems for hydrogen sensitivity. Companion samples are often retained for diffusible hydrogen content analysis.
Testing Items
Typical test items and deliverables for a hydrogen embrittlement sustained load tensile testing program include:
- Notched tensile strength (NTS) determination to establish the 100% load baseline for the specimen lot
- Sustained load verification at 75% of notched tensile strength held for 200 hours (standard lot acceptance condition)
- Time-to-failure measurement at elevated load levels (75%, 80%, 85%, 90% of NTS) for severity ranking and process comparison
- Threshold sustained stress determination through load-stepping or staircase programs
- Verification of hydrogen embrittlement relief (baking) effectiveness in accordance with ASTM B850
- Diffusible hydrogen content measurement by inert gas fusion or thermal desorption analysis as a supporting item
- Scanning electron microscope (SEM) fractography to confirm the intergranular fracture morphology characteristic of hydrogen embrittlement
Testing Methods
The test is performed in a defined sequence designed to reproduce production conditions exactly and to reveal any delayed fracture tendency:
- Specimen preparation: specimens are machined, heat treated to the specified high-strength condition, and notched under controlled conditions; hardness and microstructure are verified.
- Process exposure: specimens are subjected to the complete production processing cycle—degreasing, acid activation or pickling, electroplating (zinc, zinc-nickel, cadmium, chromium, etc.), and any conversion coating—using identical line parameters and bath chemistry as the parts represented.
- Hydrogen relief baking: within the specified time window after plating, specimens are baked in accordance with ASTM B850 or AMS 2759/9, typically at approximately 190 to 220 °C for a duration of roughly 4 to 24 hours, with longer times required for higher-strength material.
- Baseline characterization: the notched tensile strength is established on reference specimens from the same lot to define the applied load basis.
- Sustained loading: specimens are installed in calibrated dead-weight, lever-arm (creep-rupture type), spring-loaded, or servo-hydraulic sustained load frames, and the load is applied smoothly to the specified level, typically 75% of NTS, as soon as practicable after baking.
- Dwell and monitoring: the constant load is maintained for 200 hours (100 to 200 hours for certain fastener process-control programs) at room temperature, typically 20 to 25 °C, with continuous monitoring for fracture and recording of any time-to-failure.
- Evaluation: specimens that survive the full dwell without fracture or crack initiation satisfy the acceptance criterion; failed specimens are examined fractographically, and the process (pickling severity, plating bath, bake time/temperature) is investigated and corrected before requalification.
For characterization programs, multiple load levels are run to construct a stress versus time-to-failure curve, from which the hydrogen embrittlement threshold stress for the material-process combination can be estimated.
Applications
Hydrogen embrittlement sustained load tensile testing is applied wherever high-strength steel components are exposed to hydrogen-introducing processes or hydrogen-containing service environments:
- Aerospace structures: qualification of cadmium, zinc-nickel, and chromium plating on landing gear, engine mounts, actuator rods, and high-strength airframe bolts made from 4340, 300M, and 4130 steels
- Fastener manufacturing: process control and lot acceptance for electroplated property class 10.9 and 12.9 bolts, screws, and studs under ASTM F1940, ASTM F519, EN ISO 4042, and GB/T 3098 series requirements
- Automotive engineering: validation of zinc and zinc-alloy coated high-strength chassis, suspension, and powertrain fasteners, springs, and shafts
- Oil and gas industry: assessment of components for sour (H2S) service in conjunction with NACE MR0175 / ISO 15156 and NACE TM0177 sustained load practices
- Surface treatment industry: initial qualification and periodic revalidation of plating lines, pickling baths, and baking furnaces
- Metallurgical research: comparative ranking of steel grades, heat treatments, coating systems, and hydrogen charging conditions
FAQ
Why is the standard dwell time 200 hours?
Experience shows that hydrogen-induced delayed fractures under sustained load almost always occur within the first 100 to 200 hours after loading, because the diffusible hydrogen is gradually depleted or trapped as it drives crack initiation. A 200-hour hold therefore captures the critical delayed-failure window while remaining practical for production schedules.
Which materials require this test?
Hardened steels with a tensile strength of approximately 1000 MPa (about 32 HRC) or higher are considered susceptible, and many specifications mandate testing above this threshold. Typical materials include AISI 4130, 4140, 4340, 300M, H-11, maraging steels, and high-strength fastener grades.
What are the pass and fail criteria?
The standard acceptance criterion is no fracture of the notched specimen at 75% of its notched tensile strength sustained for the full 200-hour dwell. Any fracture, or evidence of crack initiation at the notch root, constitutes failure of the process and requires investigation and corrective action.
How is this test different from a regular tensile test?
A regular tensile test applies rapidly increasing load to failure and cannot reliably reveal hydrogen embrittlement. The sustained load test applies a constant subcritical load for a long duration, giving absorbed hydrogen the time needed to diffuse to the notch root and cause delayed intergranular cracking.
Can baking always remove the absorbed hydrogen?
Post-plating baking at approximately 190 to 220 °C substantially reduces diffusible hydrogen, but its effectiveness depends on strength level, coating type, bake delay, and duration. Baking parameters must therefore be validated by sustained load testing per ASTM F519 rather than assumed, as specified in ASTM B850 and AMS 2759/9.
Does this test cover environmental hydrogen embrittlement?
ASTM F519 addresses internal hydrogen embrittlement introduced by processing. For hydrogen absorbed from service environments such as wet H2S, cathodic protection, or high-pressure hydrogen gas, complementary sustained load practices such as NACE TM0177 and ASTM G142 are used.
