Effects of S50C Material Properties and Machining
Parameters on Surface Roughness
Material Behavior (S50C Properties): S50C is a medium‑carbon steel (~0.50% C) that in the annealed
condition has a coarse pearlite–ferrite microstructure. Coarse lamellar pearlite (achieved by full annealing)
yields good machinability: “coarse lamellar pearlite to coarse spheroidite microstructure gives optimum
machinability in C1050 (S50C)” 1 . In this soft state its hardness is relatively low (around 13 HRC converted
from Brinell) 1 . However, S50C can be heat-treated to high hardness (up to ~58 HRC) 2 . Higher material
hardness increases cutting resistance, vibration and friction during milling, which tends to raise surface
roughness (Ra) 3 . In other words, a harder or work‑hardened S50C surface will generally machine to a
coarser finish than the same part in the annealed state. In practice, S50C’s machinability is rated moderate
(relative index ~60%, versus 100% for a free-cutting steel) 4 , meaning it can achieve good finishes but tool
and parameter selection must be optimized. The balance of strength and ductility in S50C (versus lowercarbon steels) also affects its cutting behavior: increased strength/density tends to increase cutting forces
and potential chatter, again influencing Ra.
Machining Conditions (Roughing vs Finishing Parameters): In end‑milling, roughing operations
deliberately use aggressive parameters (high feed per tooth, large depth of cut, heavier tool engagement)
to remove material quickly, accepting a coarse surface. Finishing passes then use much lower feed rates,
shallower cuts, and often higher spindle speeds to refine the surface. Key parameter effects (supported by
literature) include:
• Feed Rate: Increasing feed (feed per tooth) produces larger scallops or cusps on the milled surface,
raising Ra. Studies on S50C show that higher feed rates “produced poorer surface quality” 5 . Thus,
doubling the feed nearly doubles the cusp height, making surfaces rougher. In practice, roughing
feeds are several times higher than finishing feeds, so roughing typically yields much higher Ra.
• Cutting Speed (Spindle Speed): Higher cutting speed tends to lower Ra for steel. The cited study on
hardened S50C found “higher cutting speed…cause[s] better surface quality” 5 , and other
machining reports agree that Ra decreases as cutting velocity increases 6 . Faster speeds reduce
built-up edge and improve chip formation, smoothing the finish. Finishing passes often use higher
speeds (within tool limits) than roughing passes for this reason.
• Depth of Cut: Deeper cuts increase cutting forces and tool deflection/vibration. Large depths (typical
in roughing) can worsen stability and thus increase Ra, whereas shallow depths in finishing minimize
these effects. (Reportedly, depth of cut has less influence on Ra than feed and speed, but very heavy
cuts can still degrade finish by inducing chatter.)
• Tool Wear and Edge Condition: As a tool wears (flank wear, chipping, built‑up edge), its sharpness
degrades and it rubs more than cuts. This “degrades” surface finish, raising Ra 7 . In milling S50C, if
one tool is used for roughing then finishing, the wear accumulated during roughing can cause the
finishing cut to be rougher than ideal. Likewise, tool material and coating choice (carbide, coatings,
etc.) affect wear rate in medium-carbon steel.
In summary, roughing parameters (high feed, high depth, lower speed) lead to high Ra, while finishing
parameters (low feed, shallow cut, higher speed) produce low Ra. Empirical guidelines support this: e.g. to
1
achieve Ra below ~0.1–0.2 µm, one typically uses very small feeds (on the order of 0.02–0.05 mm/tooth) and
fine depths.
• Roughing vs Finishing: Roughing endmills often have more flutes and positive geometry to hog
material, but these generate larger scallops on the surface. Finishing endmills have smaller corner
radii and finer geometry, which coupled with lighter passes yield smoother profiles. For S50C,
literature indicates roughing passes on hardened stock can result in Ra in the range ~0.5–0.8 µm or
higher, whereas fine finishing can reach Ra values well below 0.1 µm 5 6 .
Surface Finish Outcomes (Analysis of Provided Data): The provided Ra data for 9 samples show a wide
range (~0.043 to 0.666 µm average Ra). These differences align with the above trends. For example, sample
V4 (Ra ≈0.666 µm) is by far the roughest finish. This suggests V4 was a roughing cut with the most
aggressive combination (highest feed and depth), in line with the notion that “poor surface quality was
produced by higher feed rate” 8 . By contrast, samples V2, V3, V6–V9 all have very low Ra (~0.043–0.073
µm). These likely represent finishing passes with very low feed and shallow cuts; high cutting speed and
minimal tool engagement produced the smoothest surfaces. This matches the literature: higher speeds and
lower feeds give better finish 5 6 . The intermediate Ra values (around 0.20–0.26 µm for V1 and V5)
might be semi-finish or moderately-fed cuts – heavier than fine finishing but lighter than full roughing.
Other factors may also explain variations. For instance, if any pass encountered a localized harder
microstructure or previous work-hardening, the same settings would yield higher Ra (due to increased
cutting resistance) 3 . Also, if tool wear were significant by the time of a pass, that could elevate Ra (since
“when the surface roughness…degrades, that could indicate…tool wear” 7 ). In practice, strict control is
used: roughing and finishing are often done with different tools or fresh inserts to avoid wear effects.
Overall, the data-backed reasoning is clear: the highest Ra (V4) came from the most aggressive cut (high
feed/depth), and the lowest Ra (V2–V3, V6–V9) came from fine finishing conditions, exactly as expected from
S50C’s machinability and the known influence of feed, speed, and tool condition 5 6 .
Sources: The above analysis is grounded in metallurgical and machining research on S50C and steels in
general 5 1 2 6 7 3 , which consistently report that material hardness and cutting parameters
(feed, speed, depth, wear) dictate the surface finish in milling.
S50C | AISI 1050 | 760 Plate | Medium Carbon Steel Plate Carbon Steel Malaysia, Selangor, Kuala
Lumpur (KL), Klang Supplier, Suppliers, Supply, Supplies | E STEEL SDN. BHD.
1
https://m.ecarbonsteel.com/index.php?ws=showproducts&products_id=3172073&cat=Carbon-Steel
2
What is S50C Steel-ZGT
https://zgtsteel.com/what-is-s50c-steel/
3
CNC Machining Surface Roughness: A Complete Guide
https://geomiq.com/blog/cnc-machining-surface-roughness-guide/
4
Visão geral das propriedades do aço S50C e principais aplicações – Metal Zenith
https://metalzenith.com/pt/blogs/steel-properties/s50c-steel-properties-and-key-applications-overview
5
8 The Factors Influenced to Surface Finish in Milling for Hardened Medium Carbon Steel: JIS S50C |
Scientific.Net
https://www.scientific.net/AMR.383-390.7133
2
6 Machinability Investigations Based on Tool Wear, Surface Roughness, Cutting Temperature, Chip
Morphology and Material Removal Rate during Dry and MQL-Assisted Milling of Nimax Mold Steel
https://www.mdpi.com/2075-4442/11/3/101
7
Tool Wear Patterns in Machining | Seco Tools
https://www.secotools.com/article/122073?language=en
3