Mechanical Properties changes, Delta Ferrite in 416 Stainless 1

[Helicopterjunky]

Has anyone run across any good information on mechanical or fracture property effects of large amounts of delta ferrite in the 400 stainless series materials, particularly 416SS?
This is not a welded application as so many of the ferrite affects literature is about.

 

[metengr]

Well, for starters, there is a paper related to delta ferrite and hot workability for 416 ss;

http://www.ingentaconnect.com/content/els/09215093/2003/00000351/00000001/art01740

Regarding notch toughness, I would presume that since sulfur inclusions would have a greater impact (no pun intended) on the fracture behavior of 416 ss, having delta ferrite stringers really would not matter as such.

 

[metengr]

The influence of delta ferrite in the AISI 416 stainless steel hot workability

P. H. S. Cardosoa, C. Kwietniewski, b, J. P. Portoa, A. Regulya and T. R. Strohaeckera
a Post-Graduate Program in Metallurgical, Mining and Materials Engineering—PPGEM, Av. Osvaldo Aranha, 99/610, Porto Alegre 90035-190, Brazil
b University of Vale do Rio dos Sinos—UNISINOS, Av. Unisinos 950, Sao Leopoldo 93022-000, Brazil
Received 11 April 2001; revised 2 July 2001. Available online 27 March 2003.



Abstract
Delta ferrite in martensitic stainless steels may have an adverse effect on the mechanical properties of these materials at high temperature. The occurrence of such phase is determined by the material chemical composition (mainly Cr and C), as-received microstructure condition and hot working temperature. The aim of this investigation is to assess the influence of delta ferrite on the hot workability of the martensitic AISI 416 stainless steel. Hence, different heats of this material (differing in chemical composition and as-received microstructure) were submitted to heating tests in order to observe the microstructural transformations that take place at high temperature and then examine the influence of these transformations on the mechanical behaviour. Phase characterisation and quantification were carried out using scanning electron microscopy/energy-dispersive X-ray microanalysis and image analysis. The heating tests were performed in the temperature range of 1100–1350 °C and hot workability in two heats with different delta ferrite content was assessed by hot torsion tests in the temperature range of 1000–1250 °C. The results have indicated that chemical composition and as-received microstructure strongly affect delta ferrite formation, which in turn deteriorates hot workability of the martensitic AISI 416 stainless steel.

 

[Helicopterjunky]

Good Article summary. I will dig up the full article. Let me explain in more detail. Delta Ferrite in 416SS is often desirable because of good chip breaking especially in screw machine work. Mills will try intentionally to have a certain amount of delta ferrite existing in the material for this reason. Chip beaking means a brittle snap of a short chip versus the real ductile materials which will form curly stringy chips all around the tool holder. I have a bolt which has fractured under high torqueing. When examined it has ridiculous amount of delta ferrite, maybe 40% of grip cross section. The delta ferrite is elongated in the axis of the bolt and fracture initiated from surface of the bolt thread undercut as might be expected from overtorqueing. However, the fractography is noticably different in the delta ferrite zones, more step like, cleavage than ductile overload. The bolt operates at room temperature to slightly subzero, -20F. I am interested if crack formation at the surface can occur at lower tension/torque when delta ferrite stringers are existing at the surface instead of a uniform martensitic structure.

 

[EdStainless]

Is this material Q&T? If so how do you end up with delta ferrite unless someone botched the quench?
Is this very low C material?

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[Helicopterjunky]

The bolt exhibited both elongated delta ferrite and Sulfide inclusions. The chemistry is C/.07, Mn/.22, Si/.56, P/.02, S/.24, Cr/13.1, Mo/.07, Ni/.25. This gives a Mn/S ratio of less than 1. The bolt is heat treated, and maybe tempered in the embrittling temperature range, having a hardness of 28-29R/C. I understand from the literature that Chromium and Carbon concentration, unbroken chemical segregation in the rolled product and heat treatment can affect the amount of delta ferrite in this material. The sulphides in this bolt are predominantly Mn-Fe-Cr type where in another bolt tested with little delta ferrite the sulphides are predominantly Mn-Fe type. However the other good structure bolt with little delta ferrite has a Mn/S ratio of 6 to 1. It has sulfide stringers, but almost no ferrite. It has C/.12, Mn/1.04, Si/.37, P/.03, S/.18, Cr/12.4, Mo/.07 and Ni/.15. It too is heat treated, coming in at 27-28R/C.