Is mild steel stronger than stainless steel? Exploring the “double-edged sword” effect of material robustness

In industrial manufacturing, material robustness is often seen as the “gold standard” of performance. However, when we look at mild steel versus stainless steel, we see that there are “gains” and “losses” in robustness behind this characteristic.

Is mild steel stronger than stainless steel? Does being stronger mean it is harder to machine? This “performance-machining” paradox forces engineers to weigh up the choice of easy-to-machine mild steel or difficult-to-machine stainless steel at the design stage. This paper will explore these issues in detail.

1. Key concepts of material robustness definition

Mild steel: carbon content of 0.05% to 0.25%, iron and carbon based, may contain a small amount of manganese, silicon. Typical grades such as AISI 1018.

Stainless Steel: Contains at least 10.5% chromium and forms a passivated film to prevent rust. Common types include austenitic (304), martensitic (410), ferritic (430) and duplex stainless (2205).

Sturdiness: Usually refers to the ability of a material to resist deformation or damage and needs to be split into:

Tensile strength (the maximum stress a material can withstand before breaking)

Hardness (ability to resist localized indentation)

Toughness (ability to absorb energy and resist fracture)

2. Is mild steel stronger than stainless steel? Comparison of strength and hardness

(1) Mild steel:

Tensile strength: usually 400~550 MPa (can be raised to over 600 MPa after cold rolling).

Hardness: low (Brinell hardness of about 120 ~ 180 HB), easy to process.

Toughness: good, especially at low temperatures.

(2) Stainless steel:

Austenitic (such as 304): tensile strength of about 500 ~ 700 MPa, hardness is lower (~ 200 HB), but the strength can be significantly increased after cold working (such as 304 cold rolled up to 1000 MPa).

Martensite (such as 410): heat treatment tensile strength up to 1000 ~ 1500 MPa, high hardness (~ 50 HRC), but poor toughness.

Duplex stainless steel (e.g. 2205): both high strength (620~900 MPa) and corrosion resistance.

Conclusion:

Is mild steel stronger than stainless steel? In general, mild steels are usually less strong than martensitic or cold-worked stainless steels, but stronger than austenitic stainless steels when untreated.

In addition, stainless steel can achieve higher strength through alloy design and processing.

3. Analysis of the impact of material robustness on processing and application - comparing mild and stainless steels

The “robustness” of a material usually refers to its strength, hardness, toughness and other comprehensive mechanical properties. The impact of robustness on processing and application shows a significant bidirectionality: materials with high robustness are often difficult to process, but the application of high loads, harsh environments have significant advantages. The following two aspects from the processing characteristics and application scenarios, compare the differences between mild steel and stainless steel:

(1) Comparison of processing characteristics: the positive correlation between robustness and processing difficulty

Typical processing difficulty analysis

1) Stainless steel “work hardening trap”

Austenitic stainless steel in cutting or stamping, the surface will be rapidly hardened due to plastic deformation (hardness increase of 50% -100%), resulting in rapid tool wear (such as machining 304, carbide tool life of only 1/5 of machining mild steel). Coated tools (e.g. TiN coatings) + cooling lubricants are required to mitigate this, but at an increased cost.

2) “Strength Ceiling” Limitations of Mild Steel

Mild steel can only be cold worked to a maximum strength of about 600MPa (e.g., cold drawn steel wire), and plasticity is greatly reduced; whereas stainless steel (e.g., 17-4PH precipitation hardening) can be heat treated to a strength of 1300MPa or more, while maintaining a certain degree of toughness, which makes it suitable for high-strength parts in aerospace.

(2) Comparison of application scenarios: synergy of robustness and environmental adaptability

1) High load/high stress scenarios

● Stainless steel advantage scenarios:

Mechanical structural parts: martensitic stainless steel (such as 420SS) to make gears, shafts, hardness of 50HRC or more, wear life is 3-5 times that of mild steel (surface hardness of 55HRC after carburization, but the heart is soft).

Pressure vessel: 316L stainless steel to make high-pressure reactor (pressure ≥ 10MPa), can withstand strong acid medium corrosion; mild steel need to be lined with anti-corrosion layer, but high pressure is easy to fall off and cause leakage.

● Limitations of mild steel:

Only applicable to low load structure (such as ordinary construction steel frame, bicycle frame), if used for heavy machinery (such as crane boom), need to thicken the cross-section, resulting in excessive weight.

2) Strength retention in corrosive environments

● Stainless steel core value:

In the marine environment, duplex stainless steel (such as 2205) pitting resistance equivalent (PREN) ≥ 32, strength in service after 20 years still maintains 90% of the initial value; mild steel, if you do not do corrosion prevention, the intensity of corrosion within 5 years due to corrosion decreased by more than 50% (such as harbour guardrail need to be painted annually, otherwise rust through).

● Typical case:

If mild steel is used for the columns of an oil platform, a cathodic protection system is required, and the average annual maintenance cost accounts for 15% of the initial investment; if stainless steel is used instead, the whole life cycle cost is reduced by 40%.

3) Performance stability under extreme temperature

● High temperature scenario:

310S stainless steel (high temperature resistance 1200 ℃) for boiler piping, strength at 800 ℃ is still up to 250MPa; mild steel at 450 ℃ above the occurrence of oxidative decarburization, the strength of the plummeted to below 100MPa, which may lead to pipeline rupture.

● Low temperature scenario:

304L stainless steel in the LNG tank at - 196℃, impact toughness ≥100J, not brittle; mild steel at this temperature is brittle, may cause the tank to crack due to small impact.

4）Surface properties and functional requirements

● Sanitary field:

The smooth surface (Ra≤0.8μm) of stainless steel (e.g. 316L) inhibits bacterial adhesion and is used for medical scalpels; mild steel, even if chromium-plated, may still harbor bacteria in its surface pores and does not meet medical standards.

Aesthetics and durability:

Stainless steel curtain wall use 20 years is still bright as new; mild steel sprayed aluminum curtain wall, 5 years after the coating chalking flaking, need to be re-coated.

(3) the underlying logic of material selection: performance - cost - the scene of the triangle of balance

1) The contradictory unity of processing and application

● The “cost-effective range” of mild steel:

When the scene demand is low strength, non-corrosive environment, and complex molding (e.g., daily hardware, appliance housings), its ease of processing and low cost (about 1/3 of the price of stainless steel) become the core advantages.

● “Non-compromise scenarios” for stainless steel:

When the scenario involves corrosion, high temperature, high load, hygiene and safety (e.g. nuclear power plants, deep-sea probes, food production lines), stainless steel must be chosen even if the processing cost is high - because the early failure of mild steel can trigger catastrophic consequences (e.g. environmental accidents caused by chemical pipeline leakage, the damage can be up to 100 times the material cost (more than).

2) The limited compensatory surface treatment

Mild steel can be carburized, galvanized, sprayed and other surface treatment to enhance the local performance, but there is an intrinsic defect:

The thickness of the carburized layer is only 0.5-2mm, the core is still easy to corrode after wear;

The galvanized layer dissolves quickly in strong acid environments;

Plastic spraying layer has microporous, long-term use may be due to thermal expansion and contraction cracking, triggering corrosion of the substrate.

In contrast, the overall corrosion resistance and high strength of stainless steel is “endogenous”, without relying on acquired protection.

4. Summary: the robustness of the “double-edged sword” effect

This article provides a detailed answer to the question “Is mild steel stronger than stainless steel? This is a very doubtful question, but also based on this question to explore the “double-edged sword” effect on the robustness of metal materials, that is, materials with high robustness are often more difficult to process, but have significant advantages in the application of high loads, harsh environments.

The following are the core conclusions of this paper:

Mild steel is a “good-for-nothing” base material that provides for everyday productivity;

Stainless steel is a strategic material with “performance underpinning”, exploring the boundaries of production in extreme environments.

The two are not substitutes, but together form a material gradient from “cost-effective” to “high-performance” to meet the diverse needs of industry and life.