Electroless Nickel Heat Treatment: Hardness, Temperature, and Time
Table of Contents
Electroless Nickel Heat Treatment: Hardness, Temperature, Time
Heat treatment increases the hardness of electroless nickel phosphorus by forming nickel phosphide (Ni₃P) as the deposit separates into phases and crystallizes. The highest hardness usually comes from about one hour between 350 and 400 °C, per ASTM B733-22 Table 3 and the 1983 paper, written in 1980 and revised in 1983. The result depends on phosphorus content and the treatment cycle. Heating past the peak lowers hardness.
In the 1983 paper’s tests, deposits measured about 500 to 600 HV100 as deposited and up to 1,100 HV100 after heat treatment. ASTM B733 specifies Class 2 heat treatment at 260 to 400 °C to a minimum of 850 HK100. The 1983 values are Vickers microhardness at 100 gf (HV100), and ASTM B733 states its Class 2 minimum in Knoop at 100 gf (HK100). The two scales are not interchangeable, so a Vickers result is not checked against a Knoop minimum without a conversion basis agreed with the customer.
Hardening can reduce both corrosion resistance and ductility, as the 1983 paper’s tests showed. Heat treatment also makes the deposit more magnetic, which matters when thickness is checked with a magnetic gauge. A conventional low temperature hydrogen relief bake on steel is separate from a hardening cycle.
📈 How Hard Does Electroless Nickel Get After Heat Treatment?
In the 1983 paper’s tests, the peak was 1,050 to 1,100 HV100 after 400 °C (750 °F) for 1 hour, from about 500 to 600 HV100 as deposited. The schedules below are Vickers microhardness at 100 gf (HV100) from those tests.
Vickers Hardness (HV100) of Electroless Nickel After Heat Treatment, 1983 Test Data
Vickers microhardness at 100 gf (HV100) from the 1983 paper's tests, taken from more than one deposit and test series. These values are not acceptance limits or a universal hardness curve.
| Condition | Reported hardness, HV100 | What it shows |
|---|---|---|
| As deposited | 500 to 600 (480 to 500 for a 10 to 11% phosphorus deposit) | The starting point before any heat |
| 190 °C (375 °F) bake, typical of hydrogen relief | 500, against 480 as deposited | Little hardening |
| 290 °C (550 °F) for 2 h | 880 | Partial hardening at a lower temperature |
| 290 °C (550 °F) for 10 h | 970 | More time at the same temperature |
| 290 °C (550 °F) for 16 h | 1,050 | Near full hardness at a lower temperature |
| 340 °C (650 °F) for 4 to 6 h | 950 to 1,000 | A common hardening schedule |
| 400 °C (750 °F) for 1 h | 1,050 to 1,100 | Near the peak in these tests |
| 540 °C (1,000 °F) for 1 h | 750 | Past the peak and softer again |
⚙️ Why Does Heat Treatment Harden Electroless Nickel?
Electroless nickel hardens on heating because the as plated coating is a metastable alloy, a deposit that has not reached equilibrium, and heat lets it separate into nickel and nickel phosphide. Electroless nickel from a hypophosphite bath is nickel carrying several percent phosphorus, and ASTM B733 states that the deposit’s properties vary primarily with its phosphorus content and its later heat treatment. In the 1983 paper, deposits up to about 5% phosphorus are crystalline nickel with phosphorus in solid solution, deposits between about 5 and 8.5% are partly crystalline, and deposits above about 8.5% are amorphous, with no crystal pattern on X-rays. ASTM B733 describes low phosphorus coatings of 2 to 4% as microcrystalline, made of very small crystals, with an as plated hardness of 620 to 750 HK100.
Electroless nickel plates without a rectifier. The phosphorus content that shapes the coating’s response to heat is set in the tank.
None of those as plated structures is in equilibrium. At equilibrium, nickel dissolves almost no phosphorus, so heat gives the alloy a way to separate. The 1983 paper lays out the sequence:
- From about 220 to 260 °C (420 to 500 °F), Ni₃P precipitation begins. Small nickel phosphide particles form inside the deposit and resist deformation under an indenter. They appear first as coherent particles, still aligned with the surrounding metal, and then as distinct particles.
- Above about 320 °C (600 °F), the deposit crystallizes and loses its amorphous character.
- With continued heating the Ni₃P particles conglomerate into a two-phase alloy. Above about 8% phosphorus, Ni₃P becomes the matrix. In lower phosphorus deposits, nearly pure nickel is the main phase.
The 1983 paper describes the result as age hardening. In a 2024 study in Materials, Pedrizzetti and colleagues attribute the hardening of a high phosphorus coating at 400 °C to nickel crystallization and the precipitation of nanometric Ni₃P, which strengthens the deposit. Their particle-free coating contained 11.44% phosphorus by weight, was about 50 µm thick and was amorphous as deposited. Pedrizzetti and colleagues measured instrumented indentation hardness, hardness calculated from load and penetration depth, rising from 6.26 to 9.37 GPa after one hour at 400 °C, about 50%.
Past the peak, the coating softens. In the 1983 paper’s tests, one hour at 540 °C gave 750 HV100, against 1,100 HV100 after one hour at 400 °C.
🧰 What Temperature and Time Harden Electroless Nickel?
About one hour between 350 and 400 °C hardens electroless nickel, and longer times harden it at lower temperatures. ASTM B733-22 Table 3 includes 20 hours at 260 °C for Type I deposits among its published Class 2 schedules. In the 1983 paper’s tests, 2 hours at 290 °C (550 °F) gave 880 HV100 and 16 hours at the same temperature gave 1,050 HV100, while one hour at 400 °C reached 1,100 HV100. At 230 °C (450 °F) and below, the 1983 paper’s tests found only a minimal increase, and the paper notes that treatments in that range are rarely used except for hydrogen relief or adhesion.
A thermocouple clamped to the heaviest part in the load.
Table 3 of ASTM B733-22 gives these published Class 2 schedules for maximum hardness:
ASTM B733-22 Class 2 Schedules for Maximum Hardness
Published Class 2 heat treatment schedules from Table 3 of ASTM B733-22, set by phosphorus content. ASTM B733 recommends testing each alloy for the maximum hardness it can reach. These times and temperatures are the standard's schedules.
| Deposit type (phosphorus, wt %) | Temperature | Time |
|---|---|---|
| Type I (no phosphorus requirement) | 260 °C | 20 h |
| Type I | 285 °C | 16 h |
| Type I | 320 °C | 8 h |
| Type I | 400 °C | 1 h |
| Type II (1 to 3%) | 350 to 380 °C | 1 h |
| Type III (2 to 4%) | 360 to 390 °C | 1 h |
| Type IV (5 to 9%) | 365 to 400 °C | 1 h |
| Type V (10% and above) | 375 to 400 °C | 1 h |
ASTM B733 states that the time to maximum hardness varies with phosphorus content and that high phosphorus deposits may need a longer time, a higher temperature, or both. It recommends testing individual alloys for the hardness they can reach, especially at lower temperatures and longer times. ASTM B733 specifies a Class 2 minimum of 850 HK100. The canceled 1990 specification MIL-C-26074E set an 800 Knoop minimum for its Class 2 and listed a similar range in notes that are not mandatory, from 500 °F (260 °C) for 4 hours or more to 750 °F (399 °C) for 0.5 to 1 hour. The document, revision and class on the drawing set the minimum hardness and the permitted temperature, so read them before choosing a cycle.
The Substrate Limits the Cycle
A cycle hot enough to harden the coating heats everything under it. ASTM B733 warns that heat treatable aluminum alloys such as 7075 can change microstructurally and lose strength above 130 °C, and it specifies an adhesion treatment of 120 to 130 °C for those alloys. Every maximum hardness schedule in the table runs at 260 °C or more. For some ferrous basis metals, ASTM B733 recommends 260 °C for more than 20 hours to reduce the loss of surface hardness and strength, and it states that heat treatment can change the metallurgy of the base material, with leaded steel as its example. The 1983 paper adds part warpage as a reason to choose a lower temperature and a longer time. Whether a part can take a hardening cycle is a decision for whoever owns the substrate properties, made before the coating is ordered as Class 2.
Make Sure the Part Reaches Temperature
ASTM B733 requires enough time for large parts to reach oven temperature, and it warns against rapid heating and cooling of plated parts. For a relief bake, ASTM B733 requires that the time count from the moment the whole of each part reaches the specified temperature. A thin test strip follows the oven air far more closely than a thick flange. A heavy part can still be heating after a cycle timed from the controller has started, and it can then have less time at temperature than the strip later tested for hardness. Recommended practice is a thermocouple on the heaviest section, or a documented heat up allowance for that load, so the recorded time belongs to the part.
Oven Atmosphere
Above 260 °C, ASTM B733 recommends a vacuum or an inert or reducing atmosphere to prevent surface oxidation of the coating, and it lets the purchaser require one. ASTM B733 also requires that a gas containing hydrogen be avoided with high strength steel parts.
🧪 Does a Hydrogen Relief Bake Harden Electroless Nickel?
A conventional low temperature relief bake on steel raises electroless nickel hardness only slightly. That bake is a hydrogen relief and adhesion treatment, and it is separate from a hardening cycle.
Hydrogen is a by-product of the hypophosphite reaction, and Pedrizzetti and colleagues note that some of it stays trapped in the growing coating. On high strength steel, the concern is hydrogen that reaches the steel during cleaning, activation and plating. When the purchaser specifies it, ASTM B733-22 requires a relief bake for steel at 1,000 MPa or greater (300 HV10, 303 HB, or 31 HRC) and for surface hardened parts. ASTM B733-22 permits the post-coating treatments listed in Guide B850. B733 requires the bake to start within 3 hours of plating, or as soon as possible if part size or weight prevents that, and it recommends a start within 1 hour. Its Class 3 treatment, for adhesion on steel and hydrogen relief, is 180 to 200 °C for 2 to 4 hours.
Class 3 temperatures sit below the hardening range. In the 1983 paper’s tests, a 190 °C (375 °F) bake moved a deposit from 480 to 500 HV100 and caused no significant increase in its corrosion rate. Pedrizzetti and colleagues measured a larger change after 2 hours at 200 °C, about 20% in Vickers microhardness and 11% in instrumented indentation hardness, from early nickel grain growth without Ni₃P precipitation.
The other adhesion classes in ASTM B733 stay low as well. Class 4 is 120 to 130 °C for carburized steel and age hardened aluminum. Class 5 is 140 to 150 °C for aluminum, aluminum alloys that are not age hardened, copper and copper alloys, and beryllium. Class 6, the adhesion bake for titanium alloys at 300 to 320 °C for 1 to 4 hours, falls inside the hardening range.
Hardening leaves the embrittlement requirements of a specification in place. The canceled 1990 specification MIL-C-26074E required a relief bake for Class 1 and Class 2 steel parts of Rockwell C40 and above, and a sustained load test for the Class 2 parts in that hardness range. It said the Class 2 hardness can be achieved by heating within 4 hours after coating. Whether one cycle can serve both purposes is a question for the governing specification and the purchaser. Plating Adhesion Problems: Root Causes and Fixes covers hydrogen, bake timing and bake records from the adhesion side.
⚠️ What a Hardened Coating Gives Up
In the 1983 paper’s tests, hardening reduced the corrosion resistance of one deposit. The paper heat treated that deposit to represent commercial treatments and exposed it to 10% hydrochloric acid at ambient temperature.
10% Hydrochloric Acid Corrosion of One Electroless Nickel Deposit
One electroless nickel deposit in the 1983 paper's tests, after heat treatments representing commercial practice, exposed to 10% hydrochloric acid at ambient temperature. Hardness is Vickers microhardness at 100 gf (HV100). Results from one deposit in one acid test. These values are not acceptance limits or a service life prediction.
| Heat treatment | Hardness, HV100 | Corrosion rate, µm/y (mpy) |
|---|---|---|
| None | 480 | 15 (0.6) |
| 190 °C (375 °F) bake | 500 | 20 (0.8) |
| 290 °C (550 °F) for 6 h | 900 | 1,900 (75) |
| 290 °C (550 °F) for 10 h | 970 | 1,400 (55) |
| 340 °C (650 °F) for 4 h | 970 | 900 (35) |
| 400 °C (750 °F) for 1 h | 1,050 | 1,200 (47) |
The 1983 paper gives three reasons. Ni₃P takes phosphorus out of the surrounding alloy, and the remaining material is less passive. The particles set up small active-passive cells, local pairs of more active and more passive surface. The deposit also shrinks as it hardens, by up to 4 to 6% in volume, which raises tensile stress or reduces compressive stress and can crack the coating through to the substrate. In that same acid test, the deposit corroded at 15 µm per year as deposited and at 900 to 1,900 µm per year after the hardening treatments. The paper reported a similar reduction in other environments it tested. For the deposits and environments in those tests, the paper advises against hardened coatings where corrosion resistance is required.
Mechanical behavior changes with the same heating. In the 1983 paper’s tests, exposure above 220 °C (420 °F) can reduce the coating’s strength by 80 to 90% and can destroy its ductility, especially in lower phosphorus deposits. A hardened coating is harder and more wear resistant, and it is also more brittle.
Heat treatment changes how the coating looks to a thickness gauge as well. ASTM B733 does not consider coatings above 11.2% phosphorus ferromagnetic. The canceled 1990 specification MIL-C-26074E accepted magnetic thickness measurement on a magnetic base for coatings above 8% phosphorus. It stated that heat treatment increases coating magnetism and that heat treated coatings should be measured with the instrument calibrated against heat treated standards or equivalent foils. It did not recommend the magnetic method below 8% phosphorus. This magnetic guidance appears in the specification’s nonmandatory notes. The two documents draw the line at different phosphorus contents, so check the governing one before using a magnetic gauge on a deposit between 8 and 11% phosphorus. Why Calculated and Measured Plating Thickness Differ explains why two thickness numbers can disagree, and a heat treated coating read against as plated standards is one more way.
These figures come from particular deposits and test series. Phosphorus content, bath stabilizers and impurities, the substrate, part mass and the test method all move them. In the 1983 paper’s tests, codeposited lead, cadmium, tin or sulfur from bath stabilizers can matter more to corrosion resistance than phosphorus content. The numbers show the direction and rough size of each effect. Qualification of a real process still runs against the governing specification.
🔍 How Is Electroless Nickel Coating Hardness Measured?
Electroless nickel coating hardness is measured by microindentation, and the result names the scale and the load. ASTM B578 specifies Knoop hardness (HK) at 25 gf or 100 gf on a polished cross section of the coating, and ASTM E384 covers Knoop and Vickers microindentation in general. A callout such as 850 HK100 names both the Knoop scale and the 100 gf load. Knoop and Vickers numbers for the same coating differ. The canceled 1990 specification MIL-C-26074E left its phrase “equivalent Vickers” without a stated conversion, so the basis is agreed with the customer before a Vickers result is checked against a Knoop minimum.
A bench hardness tester. On a thin coating, preparation, indent position, and load decide how much of the reading comes from the substrate.
Knoop hardness is HK = 14,229 × F / d² and Vickers hardness is HV = 1,854.4 × F / d², with the load F in gf and the diagonal d in µm. Rockwell C reads the permanent increase in depth. The table turns those definitions into indentation size. The test load is one of the choices that limits how much the substrate affects the reading.
Calculated Indentation Size for Knoop, Vickers and Rockwell C
Calculated geometry from the definitions of the Knoop, Vickers and Rockwell C hardness numbers, not measured data. Depths are approximate geometric values for the hardness and load shown.
| Test | Hardness | Indentation size | Approximate depth |
|---|---|---|---|
| Knoop, 100 gf | 550 HK100 | 50.9 µm long diagonal, 7.1 µm short | 1.7 µm |
| Knoop, 100 gf | 850 HK100 | 40.9 µm long diagonal, 5.8 µm short | 1.3 µm |
| Vickers, 100 gf | 550 HV100 | 18.4 µm diagonal | 2.6 µm |
| Vickers, 100 gf | 850 HV100 | 14.8 µm diagonal | 2.1 µm |
| Rockwell C | 50 HRC | Measured by depth | 100 µm permanent increase |
A Rockwell C test on a 50 HRC part leaves a permanent depth increase of about 100 µm. ASTM B733 specifies minimum thicknesses of 13 µm for mild service and 25 µm for moderate service, and even its 75 µm minimum for severe service is thinner than that depth. The reading reports mostly the substrate. HRC values quoted for electroless nickel are conversions. The 1983 paper gives 48 to 52 HRC as roughly equal to 500 to 600 HV100.
On a polished cross section, B578 lays the Knoop long diagonal parallel to the substrate. Preparation of the specimen, placement of the indent, and the test load are what limit substrate influence. A surface reading on a thin coating can include the substrate.
Test pieces need the same care. The canceled 1990 specification MIL-C-26074E allowed separate specimens of an equivalent basis material, coated with the lot and kept with it until processing was complete, when the parts themselves did not suit a test, and for Class 2 it required a hardness test once a month on four parts or four separately coated specimens. A specimen speaks for the lot only if it saw the same oven, the same load and the same time at temperature as the parts.
📋 What to Record With a Heat Treated Lot
A hardness result can be interpreted only alongside the conditions that produced it. ASTM B733 requires the plating bath to be monitored periodically for pH, temperature, nickel and hypophosphite. It recommends replenishing nickel and hypophosphite often enough to stay between 90 and 100% of set point, so that coating properties stay consistent. The 1983 paper traces laminated deposits to layers of different phosphorus content caused by changes in pH or stabilizer content during plating. A heat treatment result that drifts from lot to lot may start in the tank, and Metal Turnovers (MTO) in Plating: Formula and Use describes how by-products accumulate as an electroless nickel bath ages.
Records to Keep With a Heat Treated Electroless Nickel Lot
Checklist of bath, substrate, cycle and test records so a hardness result, a corrosion complaint or a thickness reading can be traced to the conditions that produced it. This is a record list. It is not measured hardness data or an acceptance limit.
| Record | What to capture |
|---|---|
| Specification | Document, revision, type, class and required minimum hardness, with its scale and load |
| Deposit | Phosphorus range or type, and the bath condition when the lot plated, such as pH, nickel, hypophosphite and metal turnovers |
| Substrate | Alloy, temper or case condition, and hardness before coating |
| Timing | When plating finished and when each bake or hardening cycle started |
| Relief bake | Temperature, time at temperature and oven, kept separate from any hardening cycle |
| Hardening cycle | Setpoint, atmosphere, time at temperature measured on the part or load, oven and load description |
| Hardness test | Method, load, cross section or surface, specimen identity and basis material, result |
| Thickness test | Method, and for magnetic gauges whether the standards were heat treated |
The canceled 1990 specification MIL-C-26074E required steel parts to be hardness tested before coating. Retesting the substrate after the hardening cycle and comparing it with that number shows whether the cycle softened the part.
Implementation Tip: Record the relief bake and the hardening cycle as separate events, even when they run in the same oven on the same day.
❌ Common Mistakes With Heat Treated Electroless Nickel
- Using the hydrogen relief bake as the hardening cycle. The hardness table shows the small HV100 change after a 190 °C bake.
- Timing the cycle from the oven controller. A heavy part can still be heating when the clock starts.
- Hardening in air when the order calls for a protective atmosphere. ASTM B733 recommends a vacuum or an inert or reducing atmosphere above 260 °C.
- Checking coating hardness with Rockwell C on the part. The indenter goes far deeper than the coating.
- Hardening a coating chosen for corrosion resistance. The 10% hydrochloric acid table shows that loss for one deposit.
- Reading heat treated coating thickness against as plated standards. Heat treatment makes the deposit more magnetic.
✅ Keeping the Bath Record Beside the Heat Treat Record
Lab Wizard Cloud helps manufacturers monitor and maintain a consistent record of the conditions that influence electroless nickel coating performance. It records dated chemistry analyses, including titrations and calculated concentrations, along with chemical additions, alerts, and an audit trail. Other process and quality parameters, such as temperature, heat treatment conditions, and measured hardness, can also be recorded manually or through integrated signals, then plotted and statistically monitored alongside bath chemistry.
When a heat treated lot fails to reach its specified hardness or a coating develops corrosion problems, these records help engineers compare bath conditions, process changes, and heat treatment results to identify potential causes. By bringing this information into a shared process history, Lab Wizard Cloud supports more consistent troubleshooting, better informed decisions, and stronger traceability against the applicable specifications.
Related Resources
- Why Calculated and Measured Plating Thickness Differ. What a thickness number represents, and why it cannot show hardness.
- Plating Adhesion Problems: Root Causes and Fixes. Hydrogen, bake records and substrate heat treatment seen from the adhesion side.
- Metal Turnovers (MTO) in Plating: Formula and Use. How electroless nickel bath age is tracked and why by-products change the deposit.
External Links
- ASTM B733. Autocatalytic nickel phosphorus coatings: types by phosphorus content, the Class 2 minimum of 850 HK100, schedules for maximum hardness, adhesion classes, hydrogen relief timing and heat treatment atmosphere.
- The Engineering Properties of Electroless Nickel Coatings. Technical paper (1980, revised 1983): structural changes on heating, hardness after heat treatment, time and temperature trade-offs, corrosion after hardening, shrinkage and magnetism.
- Pedrizzetti et al., Materials 2024, 17(22), 5657. Dehydrogenation at 200 °C and crystallization at 400 °C in a high phosphorus electroless Ni-P coating, with X-ray diffraction and indentation hardness.
- MIL-C-26074E, Coatings, Electroless Nickel (1990). Class 2 hardness and heating schedule, relief bake, test specimens, hardness test methods and magnetic thickness notes.
- MIL-DTL-26074F cancellation notice. Directs former users of MIL-C-26074E to SAE AMS-C-26074.
- ASTM B578. Microindentation hardness of electroplated coatings with a Knoop indenter.
- ASTM E384. Knoop and Vickers microindentation hardness of materials.