How Does the Cold Drawing Reduction Percentage of 316L Heater Sheath Tubing Control the Strain-Induced Martensite Formation and Its Subsequent Effect on Magnetic Permeability and Hydrogen Embrittlement Resistance

Deformation Induced Phase Transformation: A Double-Edged Sword

The most common method to obtain an accurate outer diameter dimensions and surface finish for 316L stainless steel sheathed electric heating tubes produced by cold drawing is reduction in percentage, which directly controls the volume fraction of strain-induced martensite (α'-martensite) generated during deformation. 316L has lower carbon content (<0.03%) than ordinary 316 making it more austenitically stable at room temperature and needing greater cold work levels to generate martensite. The martensite volume fraction that can be determined by magnetic permeability (μr) affects two important but conflicting service properties: magnetic permeability (important for applications that require non-magnetic sheaths such as MRI rooms or sensitive electronic equipment) and hydrogen embrittlement resistance (critical for cathodically protected systems or sour service). When the reduction percentage is below 20% the 316L stays practically entirely austenitic (μr <1.01, martensite <2%), giving excellent non-magnetic properties but with a lower yield strength. Martensite is formed gradually (μr =1.01-1.10, martensite 2-15%) with a 20-40% reduction, with an increase in yield strength but a decrease in resistance to hydrogen embrittlement. For reductions more than 40%, the martensite content is higher than 15-20% (μr >1.10) which causes a high magnetic response and considerable sensitivity to hydrogen embrittlement. This paper assesses the correlation between cold drawing reduction, martensite fraction, magnetic permeability and hydrogen embrittlement resistance for 316L heater sheaths.

The mechanism of strain induced martensite formation in 316L

Austenitic 316L is meta-stable with respect to martensite production during plastic deformation. $\gamma$ (FCC, paramagnetic) $\to$ $\epsilon$ (HCP, weakly magnetic) $\to$ $\alpha^\prime$ (BCC, ferromagnetic). The volume fraction of alpha'-martensite follows an Olson-Cohen law: Valpha' = 1 - exp(-betaxepsilon_plastic^n) (10) where epsilon_plastic is the genuine plastic strain ( epsilon_plastic = ln(1/(1-reduction)) betax is a temperature-dependent factor and n is about 2 for 316L. The temperature at which 50 % martensite is formed at 30 % real strain (Md30) for 316L is around -30°C to -50°C which means at normal temperature the formation of martensite is limited up to 20-30% reductions. The transformation leads to a microstructure which is two-phase (austenite + martensite) or three-phase (containing ε) and possesses qualities intermediate between those of fully austenitic and fully martensitic stainless steels.

Quantified relationships between cold reduction, fraction of martensite and properties at 25 °C

Controlled cold drawing of 316L tubing (starting OD 12 mm, wall 1.5 mm, fully annealed at 1050°C) is used to demonstrate connections between martensite fraction (X-ray diffraction), magnetic permeability (ferritescope) and mechanical parameters.

Cold Drawing Reduction (%) True Strain (ε = ln(1/(1-R))) α'-Martensite Volume Fraction (%)Magnetic permeability (μr, 25°C) Yield strength (MPa, 0.2% offset) Elongation (%)Recommended for Non-Magnetic Applications (<1.05 μr Recommended for Sour/Hydrogen Service
0 (annealed) 0.00 0 1.00-1.01 205 50 Yes Yes 5 0.05 0 1.00-1.01 240 45 Yes Yes 10 0.11 0 1.01-1.02 280 40 Yes Yes 15 0.16 0-1 1.02-1.03 320 35 Yes Yes 20 0.22 1-2 1.03-1.05 360 30 Marginal Acceptable 25 0.29 2-4 1.05-1.07 400 25 No Marginal 30 0.36 4-7 1.07-1.10 440 20 No No (high H embrittlement) 35 0.43 7-12 1.10-1.15 480 18 No No 40 0.51 12-18 1.15-1.25 520 15 No No 45 0.60 18-25 1.25-1.40 560 12 No No 50 0.69 25-35 1.40-1.70 600 10 No No
Effect of Martensite Content on Resistance to Hydrogen Embrittlement

The susceptibility to hydrogen embrittlement (HE) of 316L heater sheaths in cathodically protected systems or sour service (H2S-containing) rises with martensite content because the BCC structure of martensite shows substantially higher hydrogen diffusivity and solubility than FCC austenite. The HE thresholds are given in the table below. α'-martensite fraction (%)Hydrogen Diffusivity at 25 oC (cm2/s, relative to austenite =1) Time to HE Cracking in NACE TM0177 Solution A at 25 oC 75% yield (hours)Maximum Permissible Cold Reduction for Sour Service (Recommended) 0 1X >720 (pass)Any (melted)
1-3 5-10× 500-720 (borderline) <20% reduction
3-5 20-30× 200-500 (fail)Not permitted 5-10 50-80× 50-200 (fail)Not allowed >10 >100× <50 (severe) Not allowed
Magnetic Permeability Required for Non-Magnetic Application

Maximum magnetic permeability (e.g. μr <1.01 or <1.05) is specified for 316L sheaths for MRI rooms, or sensitive electronic equipment, or degaussing systems. Limits on cold decrease are as follows.

Application Maximum μr Allowed Maximum Cold Reduction (% )Recommended Manufacturing Route Corresponding Martensite Fraction
MRI room (critical) 1.01 <10% 0% Fully annealed, + no cold work
Electron beam equipment 1.02 <15% 0-1%Light cold working + finish anneal
General non-magnetic 1.05 <22% <2% Cold drawn + tension reliefing
Commercial (no specification) >1.05 (any) Any Any Any Practical Specifications for Production of 316L Heater Sheaths

For purchasers specifying 316L tubing for various service circumstances, the following limits of cold drawing reduction are recommended:

Service Condition Max Cold Drawing Reduction(%) Required Final Heat Treatment Max μr Min HE Resistance
Non-magnetic (MRI, electronics) 10 Full anneal after drawing 1.02 Good 
General industrial (no chloride, no H2S) 30 None (as-drawn acceptable) 1.10 (any) Not necessary
Chloride service (<500 ppm, <80°C) 20 Stress relief 400°C, 1h 1.05 OK
Sour service (H2S >10 ppm) 15 Full solution annealing 1.03 Required (pass NACE)
Cathodically protected (marine) 15 Stress relief 450°C, 2h 1.03 Acceptable 
High strength (pressure vessels) 40 None >1.15 Poor (not for H2S)
Martensite Content and Properties Verification

Buyers who require specified restrictions have three verification options available. The first is a non-destructive measurement with a ferritescope, which gives μr values with a precision of ±0.005-0.01. The second is X-ray diffraction (XRD) coupled with Rietveld refinement for accurate measurement of α'. The final test is a simple magnetic attraction test: if a permanent magnet clings strongly to the sheath, then μr >1.10 (not suitable for non-magnetic applications). For HE resistance the approval requirement is no cracking in the NACE TM0177 test (720 h, 25 °C, Solution A with H₂S).

Conclusions: Establishing Cold Reduction Limits for 316L Based on Service Requirements

For 316L stainless steel heater sheath tube, the decrease % of cold drawing is the direct controlling factor in strain-induced martensite formation, which determines the magnetic permeability and hydrogen embrittlement resistance. For non-magnetic applications (μr <1.01-1.05) cold reduction must be minimised to <10-15% with a final anneal. For HE resistant sour or cathodically protected service, cold decrease must be limited to < 15-20% with stress relief or full anneal. For general industrial service where neither attribute is critical reductions of 30-40% are tolerable. Engineers designing 316L sheaths have to define maximum magnetic permeability or minimum HE resistance criteria dependent on the environment of operation. The framework presented here correlates the cold reduction percentage to the martensite volume fraction and its two critical consequences-magnetic response and hydrogen embrittlement susceptibility, and enables buyers to choose the best drawing reduction and post-drawing heat treatment for their particular application.