diff --git a/OpenNest.Tests/CincinnatiCIFiber/CIFiberMaterialNameTests.cs b/OpenNest.Tests/CincinnatiCIFiber/CIFiberMaterialNameTests.cs
new file mode 100644
index 00000000..1c78b415
--- /dev/null
+++ b/OpenNest.Tests/CincinnatiCIFiber/CIFiberMaterialNameTests.cs
@@ -0,0 +1,63 @@
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Text.Json;
+using System.Text.Json.Serialization;
+using OpenNest.Posts.CincinnatiCIFiber;
+
+namespace OpenNest.Tests.CincinnatiCIFiber;
+
+/// <summary>
+/// Steel is named "Carbon Steel" across OpenNest (the Cincinnati post's libraries and
+/// the company material list), so the CI Fiber post must list it explicitly rather
+/// than reach MSN only through DefaultMaterialCode. "Mild Steel" stays for
+/// nests that already use it.
+/// </summary>
+public class CIFiberMaterialNameTests
+{
+    private static CIFiberPostConfig LoadShipped()
+    {
+        var dir = new DirectoryInfo(AppContext.BaseDirectory);
+        while (dir != null && !File.Exists(Path.Combine(dir.FullName, "OpenNest.sln")))
+            dir = dir.Parent;
+        Assert.NotNull(dir);
+
+        var path = Path.Combine(
+            dir!.FullName,
+            "Posts",
+            "OpenNest.Posts.CincinnatiCIFiber",
+            "OpenNest.Posts.CincinnatiCIFiber.json"
+        );
+        var options = new JsonSerializerOptions { Converters = { new JsonStringEnumConverter() } };
+        return JsonSerializer.Deserialize<CIFiberPostConfig>(File.ReadAllText(path), options)!;
+    }
+
+    public static IEnumerable<object[]> Configs()
+    {
+        yield return new object[] { "shipped json" };
+        yield return new object[] { "built-in default" };
+    }
+
+    private static CIFiberPostConfig For(string source) =>
+        source == "shipped json" ? LoadShipped() : new CIFiberPostConfig();
+
+    [Theory]
+    [MemberData(nameof(Configs))]
+    public void CarbonSteel_IsListedAsMSN(string source)
+    {
+        var codes = For(source).MaterialCodes;
+
+        Assert.True(codes.TryGetValue("Carbon Steel", out var code), $"{source}: no Carbon Steel entry");
+        Assert.Equal("MSN", code);
+    }
+
+    [Theory]
+    [MemberData(nameof(Configs))]
+    public void MildSteel_StillMapsToMSN(string source)
+    {
+        var codes = For(source).MaterialCodes;
+
+        Assert.True(codes.TryGetValue("Mild Steel", out var code), $"{source}: no Mild Steel entry");
+        Assert.Equal("MSN", code);
+    }
+}
diff --git a/Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostConfig.cs b/Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostConfig.cs
index ecd087fb..aca43e1d 100644
--- a/Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostConfig.cs
+++ b/Posts/OpenNest.Posts.CincinnatiCIFiber/CIFiberPostConfig.cs
@@ -188,6 +188,7 @@ namespace OpenNest.Posts.CincinnatiCIFiber
 
         private Dictionary<string, string> _materialCodes = new(StringComparer.OrdinalIgnoreCase)
         {
+            ["Carbon Steel"] = "MSN",
             ["Mild Steel"] = "MSN",
         };
 
diff --git a/Posts/OpenNest.Posts.CincinnatiCIFiber/OpenNest.Posts.CincinnatiCIFiber.json b/Posts/OpenNest.Posts.CincinnatiCIFiber/OpenNest.Posts.CincinnatiCIFiber.json
index 5770b6b7..28370359 100644
--- a/Posts/OpenNest.Posts.CincinnatiCIFiber/OpenNest.Posts.CincinnatiCIFiber.json
+++ b/Posts/OpenNest.Posts.CincinnatiCIFiber/OpenNest.Posts.CincinnatiCIFiber.json
@@ -17,6 +17,7 @@
   "ProgramStartMacro": "PROGRAMSTART.NC",
   "ProgramEndMacro": "PROGRAMEND.NC",
   "MaterialCodes": {
+    "Carbon Steel": "MSN",
     "Mild Steel": "MSN"
   },
   "DefaultMaterialCode": "MSN"
diff --git a/docs/cincinnati-ci-fiber-post-output.md b/docs/cincinnati-ci-fiber-post-output.md
index f5daceaf..09cf984e 100644
--- a/docs/cincinnati-ci-fiber-post-output.md
+++ b/docs/cincinnati-ci-fiber-post-output.md
@@ -61,6 +61,12 @@ In the desktop app these settings open in a sectioned editor (Machine, Sheets,
 Material, Program output, Macros) with a material-code table; values are
 validated and saved only when OK is pressed.
 
+Steel is "Carbon Steel" in OpenNest, as in the Cincinnati post's cut libraries.
+The material-code table maps both "Carbon Steel" and "Mild Steel" to `MSN`. A
+deployed config is copied from the shipped JSON only when a PC has none, so an
+existing PC needs the "Carbon Steel" row added in the material-code table.
+Until then such nests still post `MSN` through `DefaultMaterialCode`.
+
 - Assign linear lead-ins before posting, including circular holes. The writer
   rejects missing or arc-first lead-ins. Its conservative rule cites the G238
   compensation-selection restriction in TF5200 §13.2.4.1; do not generalize it