Elastic Recovery Rate Testing of 631 Stainless Steel in Spring Manufacturing

Think about the springs in your car’s shock absorbers, the door hinges of a commercial fridge, or the valves in an airplane engine—they all have one job: bend, stretch, or compress, then snap back to their original shape. If a spring can’t “bounce back” (what we call elastic recovery), it’s useless. It might sag in a car’s suspension, stick in a fridge door, or fail in a critical engine valve—costing time, money, or even safety.​

That’s why choosing the right material and testing its elastic recovery rate is make-or-break for spring manufacturers. And 631 stainless steel (also called 17-7PH) is a top pick for high-performance springs. It’s a precipitation-hardening stainless steel—meaning it gets stronger with heat treatment, while keeping great elasticity. But even 631 needs to be tested: not all 631 springs perform the same, and small changes in manufacturing can kill their elastic recovery. This article breaks down how to test 631’s elastic recovery rate, what makes it perform well, and how one spring factory used these tests to fix their quality issues.​

Why 631 Stainless Steel Is a Go-To for High-Demand Springs​

Before diving into testing, let’s clear up why 631 is so popular for springs. Most springs use materials like carbon steel or standard stainless steel (304), but 631 stands out for two big reasons:​

Strength + Elasticity Balance: 631 has a yield strength of 1100–1300 MPa (way higher than 304’s 205 MPa) and an elongation rate of 15–20%. That means it can handle heavy loads (like a car’s weight on shock springs) without permanent bending, then snap back.​

Corrosion Resistance: Unlike carbon steel (which rusts fast), 631 resists moisture and chemicals. This is critical for outdoor springs (like garden equipment) or industrial springs (in chemical plants)—rust weakens springs and ruins their recovery ability.​

But here’s the catch: 631’s performance depends on how it’s processed. If you mess up the heat treatment or cold work it too much, its elastic recovery rate drops. For example, a 631 spring with poor heat treatment might only recover 80% of its shape after bending—after a few weeks of use, it’ll start to sag. That’s why testing is non-negotiable.​

What Is Elastic Recovery Rate? And Why Does It Matter for Springs?​

Elastic recovery rate is simple: it’s the percentage of a spring’s original shape it regains after being stretched or compressed (and then unloaded). For example:​

If you stretch a spring from 10cm to 15cm (5cm total deformation) and it bounces back to 10.5cm (only 0.5cm permanent deformation), its recovery rate is (5–0.5)/5 × 100% = 90%.​

Most springs need a recovery rate of 90% or higher to last. Here’s why:​

Low recovery rate (below 85%): The spring gets “tired” fast. A car shock spring with 80% recovery might start sagging after 10.000 miles—making the ride bumpy and increasing tire wear.​

High recovery rate (92%+): The spring stays reliable for years. Aerospace springs (used in plane landing gear) often need 95%+ recovery—failure here could be catastrophic.​

For 631 stainless steel springs, the goal is 90–95% recovery. Testing makes sure we hit that mark.​

How to Test 631 Stainless Steel’s Elastic Recovery Rate (Step-by-Step)​

Testing isn’t complicated, but it needs to be precise. Here’s the standard method used by most spring manufacturers, based on ASTM E228 (the industry standard for elastic recovery testing):​

1. Prep the Sample (Get This Right First!)​

You can’t test a finished spring directly (its shape makes measurements messy)—so you use a “test coupon” (a small, flat piece of 631 stainless steel) that matches the spring’s processing. Here’s what to do:​

Cut a coupon 150mm long, 12mm wide, and 2mm thick (this size works for most spring grades).​

Make sure the coupon has the same heat treatment as the finished spring (e.g., if the spring is heat-treated at 510°C for 1 hour, the coupon should be too).​

Polish the coupon’s edges to remove burrs—burrs can cause uneven deformation and wrong results.​

A spring factory in Ohio once skipped this step: they tested a coupon with no heat treatment, got a 92% recovery rate, but the finished springs (with heat treatment) only had 83%. They wasted weeks reworking parts—all because of a bad sample.​

2. Set Up the Testing Machine​

Use a universal testing machine (UTM)—this tool stretches the coupon at a controlled speed and measures deformation. Here’s the setup:​

Clamp the coupon in the UTM’s jaws, leaving 100mm of length between the clamps (this is the “gauge length” we’ll measure).​

Set the machine to stretch the coupon at 2mm/min (slow enough to avoid sudden stress spikes).​

Attach a strain gauge (or use the UTM’s built-in sensor) to track how much the coupon stretches.​

3. Run the Test: Load, Unload, Measure​

The test has two key phases: loading (stretching) and unloading (letting it bounce back):​

Load Phase: Stretch the coupon until it reaches 70% of 631’s yield strength (about 770 MPa for fully heat-treated 631). Don’t go over yield strength—if you do, the coupon gets permanent damage, and the test is useless.​

Hold: Keep the coupon at 770 MPa for 1 minute—this mimics a spring holding a load (like a car’s weight on a shock).​

Unload Phase: Let the UTM slowly release the tension (2mm/min speed) until the load hits 0.​

Measure: Use a caliper to check the gauge length now. Compare it to the original 100mm to find permanent deformation.​

4. Calculate the Elastic Recovery Rate​

The formula is simple:​

Elastic Recovery Rate (%) = [(Total Deformation – Permanent Deformation) / Total Deformation] × 100%​

Example:​

Total deformation (stretched length – original length): 5mm (105mm – 100mm)​

Permanent deformation (unloaded length – original length): 0.3mm (100.3mm – 100mm)​

Recovery rate: [(5 – 0.3)/5] × 100% = 94% (great for most springs!)​

Key Factors That Kill 631’s Elastic Recovery Rate​

Even with good testing, 631’s recovery rate can drop if you mess up these three process steps:​

1. Heat Treatment (The Biggest Factor)​

631 needs a specific “age hardening” heat treatment to reach peak elasticity. The standard cycle is:​

Solution treat: Heat to 1065°C for 1 hour, then quench (cool fast) in water.​

Age harden: Heat to 510–540°C for 1–2 hours, then air cool.​

Mistakes here kill recovery:​

Too low age temperature (below 500°C): The steel doesn’t form enough “precipitates” (tiny particles that boost strength). Recovery rate drops to 85% or lower.​

Too high age temperature (above 550°C): Precipitates grow too big, making the steel brittle. Recovery rate might hit 88%, but the spring will crack easily.​

A factory in Indiana once used 560°C for aging—their 631 springs had a 89% recovery rate but cracked in 200 cycles of use. Fixing the temperature to 520°C pushed recovery to 93% and eliminated cracking.​

2. Cold Working (Don’t Over-Do It)​

Cold working (stretching or bending the steel at room temperature) makes 631 stronger—but too much kills elasticity. Most 631 springs use 20–30% cold work (meaning the steel is stretched 20–30% of its length during manufacturing).​

Over 40% cold work: The steel gets too stiff. Recovery rate drops to 82–85%—the spring can’t bounce back fully.​

Under 15% cold work: The steel is too soft. It might have 95% recovery, but it bends permanently under heavy loads.​

3. Stress Level During Use (Don’t Push It Too Far)​

Even a well-made 631 spring will lose recovery if you load it beyond its limits. For example, if you use a 631 spring rated for 500N in a 700N application, its recovery rate will drop from 93% to 80% after a few hundred cycles—permanent deformation sets in fast.​

Real-World Case: Fixing a Car Shock Spring Problem​

A Michigan-based spring manufacturer supplies 631 stainless steel shock springs to a small automotive company. In 2023. they got a flood of complaints: the springs were sagging after 8.000 miles. Here’s how they fixed it with testing:​

Tested the Failed Springs: They cut coupons from the saggy springs and ran recovery tests—only 83% recovery rate (way below the 90% target).​

Found the Issue: They checked their heat treatment logs—turns out, a new operator had set the age temperature to 480°C (too low) instead of 520°C. The steel didn’t form enough precipitates, so recovery was low.​

Fixed the Process: They reset the age temperature to 520°C, re-tested coupons (92% recovery), and made new springs.​

Results: The new springs lasted 30.000+ miles with no sagging. Customer complaints dropped to zero, and the automotive company increased their order by 50%.​

The factory’s quality manager said: “We used to skip testing for small batches—now we test every lot. It’s saved us from costly recalls and kept our customers happy.”​

Conclusion​

631 stainless steel is a fantastic material for high-performance springs—but its value depends on two things: good processing and thorough elastic recovery rate testing. Testing ensures your springs bounce back reliably, last longer, and don’t fail when it matters most.​

The key steps are simple: prep the right sample, use a universal testing machine, load to 70% of yield strength, and calculate recovery rate. Then, fix process issues (like bad heat treatment or over-cold working) to hit that 90–95% target.​

For spring manufacturers, this isn’t just extra work—it’s an investment. A well-tested 631 spring costs a little more upfront, but it saves money on returns, recalls, and customer loss. And in industries like automotive or aerospace, where spring failure is risky, testing is non-negotiable.​

At the end of the day, a spring’s job is to bounce back—and testing makes sure 631 stainless steel does exactly that.