316 vs 316Ti – Composition, Heat Treatment, Properties, and Applications

Introduction

Type 316 and type 316Ti are both austenitic stainless steels widely used across chemical processing, marine, food, and heat-exchanger industries. Engineers, procurement managers, and manufacturing planners commonly face a selection dilemma between slightly different corrosion performance, weldability, thermal stability, and cost — for example, choosing between a lower first-cost material with excellent general corrosion resistance and a stabilized variant intended for prolonged exposure to elevated temperatures.

The principal metallurgical difference is that 316Ti contains a deliberate titanium addition to bind carbon and reduce the risk of chromium carbide precipitation during high-temperature exposure; 316 is the unstabilized, conventional molybdenum-bearing austenitic grade. Because of that stabilization strategy, the two grades are often compared when designs must tolerate post-weld exposure, prolonged high-temperature service, or when choosing weld filler and welding process parameters.

1. Standards and Designations

• ASTM / ASME:
• 316: ASTM A240 / ASME SA240 (UNS S31600)
• 316Ti: ASTM A240 / ASME SA240 (commonly specified as 316Ti, UNS S31635 or EN 1.4571)
• EN (European):
• 316: EN 1.4401
• 316Ti: EN 1.4571
• JIS (Japan): equivalents exist (e.g., SUS316)
• GB (China): equivalents exist (e.g., 0Cr17Ni12Mo2)

Classification: both 316 and 316Ti are stainless steels (austenitic). They are not carbon steels, tool steels, or HSLA steels.

2. Chemical Composition and Alloying Strategy

The following table gives typical compositional ranges (wt%) according to common standards (ranges reflect standard specifications rather than a single proprietary analysis). Exact composition varies with specification and producer.

How alloying affects behavior: - Chromium (Cr) provides the passive film that gives stainless steels their corrosion resistance. - Nickel (Ni) stabilizes the austenitic phase and improves toughness and ductility. - Molybdenum (Mo) increases resistance to pitting and crevice corrosion in chloride environments. - Carbon (C) increases strength slightly but promotes formation of chromium carbides at grain boundaries when exposed to the sensitization temperature range (roughly 450–850°C), reducing intergranular corrosion resistance. - Titanium (Ti) in 316Ti preferentially forms stable titanium carbides and nitrides, keeping carbon from tying up chromium and thereby reducing the risk of sensitization after exposure to critical temperature ranges.

3. Microstructure and Heat Treatment Response

Microstructure: - Both 316 and 316Ti are fully austenitic (face-centered cubic) in the solution-annealed condition. Typical microstructure consists of an austenitic matrix; minor phases such as carbides (M23C6), nitrides, or intermetallics can appear depending on thermal history. - In 316, carbon can combine with chromium at grain boundaries to form chromium carbides (M23C6) when the material is slowly cooled through or held within the sensitization range, leading to chromium-depleted zones and intergranular corrosion susceptibility. - In 316Ti, titanium forms TiC and TiN preferentially during thermal exposure, limiting chromium carbide formation and helping preserve grain-boundary chromium.

Heat treatment response: - Solution annealing (typical range for austenitic grades): heat to approximately $1040$–$1150^\circ\text{C}$ and quench. This dissolves precipitates and restores corrosion resistance. - Normalizing is not generally used for austenitic stainless steels as it has little effect and does not harden the structure. - Neither 316 nor 316Ti is hardenable by conventional quenching and tempering — they work harden by cold deformation or can be strengthened by cold working or by precipitation hardening in different alloy systems (not typical for 316 family). - Thermo-mechanical processing (cold work, anneal schedules) affects grain size, texture, and mechanical properties similarly for both grades, but 316Ti retains better resistance to intergranular attack after high-temperature exposure.

4. Mechanical Properties

The properties below reflect typical solution-annealed (annealed) conditions; exact values depend on product form and mill treatment.

Explanation: - Strength and ductility are broadly similar; titanium stabilization does not substantially raise room-temperature tensile strength in annealed condition. - 316Ti can exhibit slightly higher minimum yield or hardness in some production lots due to precipitation of TiC/TiN, but differences are small in most structural design contexts. - Toughness remains high for both; neither is a low-temperature brittle material.

5. Weldability

Both grades are considered readily weldable with standard austenitic stainless welding practices, but there are practical distinctions.

Important weldability indices: - Carbon equivalent forms can be used to assess hardenability and cracking susceptibility. One standard index is: $$ CE_{IIW} = C + \frac{Mn}{6} + \frac{Cr+Mo+V}{5} + \frac{Ni+Cu}{15} $$ - A more detailed parameter for stainless steels: $$ P_{cm} = C + \frac{Si}{30} + \frac{Mn+Cu}{20} + \frac{Cr+Mo+V}{10} + \frac{Ni}{40} + \frac{Nb}{50} + \frac{Ti}{30} + \frac{B}{1000} $$

Qualitative interpretation: - Both 316 and 316Ti have low carbon relative to many structural steels, so they are not prone to cold cracking from martensite formation; austenitic stainless fillers and shielding gases are standard. - 316L (low-carbon variant) is often preferred where extensive welding is anticipated because the lower carbon removes the need for stabilization. For 316 (standard) and 316Ti: - 316Ti provides better resistance to post-weld sensitization when HAZ temperatures pass through the sensitization range, because titanium ties up carbon. This can be advantageous for fabrication where the component experiences repeated high-temperature cycles. - However, titanium in the base metal can complicate filler metal selection and requires attention to welding procedure to avoid forming coarse titanium precipitates in the weld metal that could affect ductility or corrosion resistance locally. In many welding applications, 316L filler is used to reduce the risk of sensitization in weld metal. - Pre- and post-weld solution annealing can restore corrosion resistance where required. For critical applications, tests for intergranular corrosion (e.g., ASTM A262 practices) are used.

6. Corrosion and Surface Protection

• For stainless grades like 316 and 316Ti, general corrosion resistance derives from the passive chromium oxide film; Mo improves pitting resistance in chloride-containing environments.
• Use the Pitting Resistance Equivalent Number (PREN) for comparative pitting resistance when relevant: $$ \text{PREN} = \text{Cr} + 3.3 \times \text{Mo} + 16 \times \text{N} $$
• Note: Titanium does not enter the PREN expression; adding Ti does not directly increase PREN but helps maintain chromium in solution by preventing chromium carbide formation.
• Sensitization: 316 (unstabilized) can become sensitized if exposed to the approximate 450–850°C range for sufficient time, leading to intergranular corrosion due to chromium carbide precipitation. 316Ti is specifically designed to reduce this risk via TiC/TiN formation.
• Stress corrosion cracking (SCC) susceptibility in chloride environments is a design consideration for austenitic stainless steels; lowering carbon does not prevent SCC; selection of material, control of residual stresses, and environment control are required.
• Surface protection: For non-stainless steels, common measures include galvanizing or painting; for 316/316Ti these are not typically required unless aesthetic or abrasive wear protection is needed.

7. Fabrication, Machinability, and Formability

• Machinability: Austenitic stainless steels are generally more difficult to machine than carbon steels due to work hardening and low thermal conductivity. 316 and 316Ti are similar; Ti stabilization can marginally reduce machinability in some heats because of the presence of hard TiC/TiN particles.
• Formability: Both grades have excellent ductility and can be deep-drawn, bent, and formed using standard techniques. Annealed material is preferred for forming to avoid excessive springback or strain hardening.
• Surface finishing: Both can be ground, polished, and electropolished. Titanium-bearing precipitates in 316Ti are generally microscale and do not preclude high-quality surface finishes.
• Welding and post-weld treatments: 316L filler metals are often used; where high-temperature creep or prolonged elevated-temperature exposure is expected, specialized filler that matches the base metal strategy may be selected.

8. Typical Applications

Selection rationale: - Choose 316 for general corrosion resistance, cost-effectiveness, and broad availability. - Choose 316Ti when service includes repeated or prolonged exposure to temperatures where sensitization is a concern, or where long-term thermal stability of grain boundaries is required.

9. Cost and Availability

• Cost: 316 is generally more common and slightly less costly than 316Ti because the latter includes a deliberate titanium addition and often tighter processing control. Price differentials are typically modest relative to overall project costs but can depend on market titanium prices and alloy processing.
• Availability: 316 is widely available in sheet, plate, tube, bar, and forgings. 316Ti is also widely available but may have longer lead times for some product forms, thicknesses, or specialty finishes. For large-volume standard stock, 316 will often be easier to source.

10. Summary and Recommendation

Recommendations: - Choose 316 if: you need a versatile, cost-effective austenitic stainless steel for general corrosion resistance, pump and valve components, food processing equipment, or when extensive shop or field welding will be performed and the lowest overall alloy cost is a priority (consider 316L for heavy welding). - Choose 316Ti if: the component will see prolonged or repeated exposure in the sensitization temperature range (e.g., heat exchangers, furnace parts, some petrochemical services), or where preservation of grain-boundary chromium after thermal cycling is critical. 316Ti is a preferred option when designers want a stabilized austenitic stainless without moving to a low-carbon grade.

Closing note: For any critical application, specify the exact ASTM/EN grade designation, required product form, and any post-weld heat treatment or solution annealing requirements. Corrosion testing, welding procedure qualification, and supplier certification are recommended to ensure that the selected grade meets service expectations.