Steel is an alloy based on iron that typically contains what approximate percentage of carbon by mass?

Introduction / Context:
Steel is one of the most important engineering materials and is widely used in construction, machinery, and tools. It is based on iron but gains its useful properties from the addition of carbon and sometimes other alloying elements. The percentage of carbon plays a major role in determining whether the alloy is considered steel or cast iron and also influences hardness, strength, and ductility. This question asks you to identify the typical range of carbon content in steel.

Given Data / Assumptions:

• Steel is an iron based alloy with carbon and possibly other elements.
• The options describe different ranges of carbon content by mass.
• We assume standard definitions used in materials science.
• Cast iron has higher carbon content than steel.

Concept / Approach:
By conventional definition, steel is an alloy of iron that contains a controlled amount of carbon, generally up to about 2 percent by mass. Low carbon steels may have around 0.1 to 0.3 percent carbon, medium carbon steels around 0.3 to 0.6 percent, and high carbon steels up to about 1.0 or slightly above. All of these fall well below 2 percent carbon. When the carbon content rises significantly above about 2 percent, the alloy is usually classified as cast iron rather than steel. Therefore, the correct description is that steel contains less than about 2 percent carbon by mass.

Step-by-Step Solution:
Step 1: Recall the definition of steel as an iron carbon alloy with controlled, relatively low carbon content. Step 2: Note that typical structural steels have less than 1 percent carbon, while tool steels may approach or slightly exceed 1 percent. Step 3: Remember that cast irons, in contrast, often contain between about 2 percent and 4 percent carbon. Step 4: Compare these figures with the ranges provided in the options. Step 5: Recognise that the only option matching the standard definition is that steel contains less than about 2 percent carbon.

Verification / Alternative check:
Materials science texts normally state that steels are iron based alloys with a carbon content up to about 2.0 percent, while cast irons have higher carbon levels. For example, mild steel might have about 0.2 percent carbon, and high carbon spring steel might have around 0.8 percent. None of these approach 10 percent, let alone 29 percent. Zero carbon would mean pure iron, which is relatively soft and not usually called steel in practical engineering. These standard figures confirm that the correct choice is the option describing less than 2 percent carbon.

Why Other Options Are Wrong:
The option no carbon is incorrect because carbon is essential to harden iron and make steel; pure iron has very different mechanical properties. More than about 10 percent carbon is unrealistic for steel and does not correspond to common metallurgical categories; such an alloy would not be classified as steel. More than about 29 percent carbon is even more extreme and does not match any standard iron carbon alloy; in fact, carbon solubility in iron is limited and such a composition is not meaningful for steel. These options are therefore wrong.

Common Pitfalls:
Some learners may think that since carbon has a strong effect on properties, steel must contain very high carbon percentages. Others may incorrectly assume that steel and cast iron have similar compositions. To avoid confusion, remember that small changes in carbon content within a relatively low range have big effects on hardness and strength. The key threshold is around 2 percent; below that is considered steel, above that is typically cast iron.

Final Answer:
Steel is an iron based alloy that usually contains less than about 2 percent carbon by mass.